feat(skills): skill system + load_skill tool via per-turn tools filter

Skills are markdown in .skills/ dirs (~/.skills + <cwd>/.skills, cwd shadows home;
name = filename). Format: line1 summary, line2 ---, body line3+; load strips the
first two lines; malformed = no summary but still loadable.

Mechanism (first use of the context-assembly filter chain, §3.2):
- kernel: expose HostAPI.applyFilters (delegates to bus.applyFilters)
- session-orchestrator: define/export toolsFilter + ToolAssembly; apply once per turn
  before runTurn (cache-stable across steps), threading cwd + conversationId
- skills (new ext): pure parse/merge/render + load_skill tool (live read, path-contained)
  + a toolsFilter filter rewriting load_skill's description + name enum per cwd
- host-bin: register skills in CORE_EXTENSIONS
- transport-http: fix HostAPI test stub for the new applyFilters method (fan-out)

734 vitest + 109 bun = 843 tests; tsc -b EXIT 0; biome clean; clean live boot.

5 files changed, 148 insertions, 2 deletions

diff --git a/packages/session-orchestrator/src/extension.ts b/packages/session-orchestrator/src/extension.ts
index fbb7c15..697eb4a 100644
--- a/packages/session-orchestrator/src/extension.ts
+++ b/packages/session-orchestrator/src/extension.ts
@@ -8,6 +8,7 @@ import {
 	sessionOrchestratorHandle,
 } from "./orchestrator.js";
 import { selectFirstProvider } from "./pure.js";
+import { toolsFilter } from "./tools-filter.js";
 
 export const manifest: Manifest = {
 	id: "session-orchestrator",
@@ -36,6 +37,7 @@ export function activate(host: HostAPI): void {
 			const provider = host.getProviders().get(r.providerId);
 			return provider ? { provider, model: r.model } : undefined;
 		},
+		applyToolsFilter: (assembly) => host.applyFilters(toolsFilter, assembly),
 		runTurn,
 		logger: host.logger,
 		now: () => Date.now(),
diff --git a/packages/session-orchestrator/src/index.ts b/packages/session-orchestrator/src/index.ts
index 3270c46..071b616 100644
--- a/packages/session-orchestrator/src/index.ts
+++ b/packages/session-orchestrator/src/index.ts
@@ -11,3 +11,4 @@ export {
 	generateTurnId,
 	selectFirstProvider,
 } from "./pure.js";
+export { type ToolAssembly, toolsFilter } from "./tools-filter.js";
diff --git a/packages/session-orchestrator/src/orchestrator.test.ts b/packages/session-orchestrator/src/orchestrator.test.ts
index ea564c5..dcaad7d 100644
--- a/packages/session-orchestrator/src/orchestrator.test.ts
+++ b/packages/session-orchestrator/src/orchestrator.test.ts
@@ -13,6 +13,7 @@ import type {
 import { runTurn } from "@dispatch/kernel";
 import { describe, expect, it } from "vitest";
 import { createSessionOrchestrator } from "./orchestrator.js";
+import type { ToolAssembly } from "./tools-filter.js";
 
 function createInMemoryStore(): ConversationStore & {
 	readonly data: Map<string, ChatMessage[]>;
@@ -87,6 +88,10 @@ function createFakeTool(
 	};
 }
 
+function identityApplyToolsFilter(assembly: ToolAssembly): Promise<ToolAssembly> {
+	return Promise.resolve(assembly);
+}
+
 describe("handleMessage integration", () => {
 	it("loads history, runs turn, emits events, and persists result", async () => {
 		const store = createInMemoryStore();
@@ -103,6 +108,7 @@ describe("handleMessage integration", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 		});
 
@@ -154,6 +160,7 @@ describe("handleMessage integration", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 		});
 
@@ -200,6 +207,7 @@ describe("handleMessage integration", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 		});
 
@@ -229,6 +237,7 @@ describe("handleMessage integration", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			resolveDispatch: () => ({ maxConcurrent: 4, eager: false }),
 			runTurn,
 		});
@@ -277,6 +286,7 @@ describe("handleMessage model resolution", () => {
 			conversationStore: store,
 			resolveProvider: () => fallbackProvider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			resolveModel: (name) => {
 				if (name === "cred/gpt-4") return { provider: resolvedProvider, model: "gpt-4" };
 				return undefined;
@@ -308,6 +318,7 @@ describe("handleMessage model resolution", () => {
 			conversationStore: store,
 			resolveProvider: () => fallbackProvider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			resolveModel: () => undefined,
 			runTurn: captureRunTurn,
 		});
@@ -338,6 +349,7 @@ describe("handleMessage model resolution", () => {
 			conversationStore: store,
 			resolveProvider: () => fallbackProvider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			resolveModel: () => ({
 				provider: { id: "should-not-use", stream: async function* () {} },
 				model: "x",
@@ -365,6 +377,7 @@ describe("handleMessage model resolution", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn: captureRunTurn,
 		});
 
@@ -398,6 +411,7 @@ describe("handleMessage model resolution", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn: captureRunTurn,
 			now: fakeNow,
 		});
@@ -422,6 +436,7 @@ describe("handleMessage model resolution", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn: captureRunTurn,
 		});
 
@@ -450,6 +465,7 @@ describe("turn-sealed event", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 		});
 
@@ -502,6 +518,7 @@ describe("turn-sealed event", () => {
 			conversationStore: wrappedStore,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 		});
 
@@ -548,6 +565,7 @@ describe("turn-sealed event", () => {
 			conversationStore: failingStore,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 		});
 
@@ -581,6 +599,7 @@ describe("turn metrics persistence", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 			now: () => 1000,
 		});
@@ -640,6 +659,7 @@ describe("turn metrics persistence", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [tool],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 			now: () => 1000,
 		});
@@ -698,6 +718,7 @@ describe("turn metrics persistence", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 			now: clock.now,
 		});
@@ -764,6 +785,7 @@ describe("turn metrics persistence", () => {
 			conversationStore: store,
 			resolveProvider: () => provider,
 			resolveTools: () => [tool],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 			now: () => 1000,
 		});
@@ -817,6 +839,7 @@ describe("turn metrics persistence", () => {
 			conversationStore: failingMetricsStore,
 			resolveProvider: () => provider,
 			resolveTools: () => [],
+			applyToolsFilter: identityApplyToolsFilter,
 			runTurn,
 		});
 
@@ -836,6 +859,102 @@ describe("turn metrics persistence", () => {
 	});
 });
 
+describe("tools filter", () => {
+	it("applies the tools filter once and passes the result to runTurn", async () => {
+		const store = createInMemoryStore();
+		const provider: ProviderContract = { id: "p", stream: async function* () {} };
+		const { captured, captureRunTurn } = createCapturingRunTurn();
+
+		const toolA = createFakeTool("tool-a", async () => ({ content: "a" }));
+		const toolB = createFakeTool("tool-b", async () => ({ content: "b" }));
+
+		let filterCallCount = 0;
+		const transformingFilter = (assembly: ToolAssembly): Promise<ToolAssembly> => {
+			filterCallCount++;
+			return Promise.resolve({ ...assembly, tools: [toolB] });
+		};
+
+		const orchestrator = createSessionOrchestrator({
+			conversationStore: store,
+			resolveProvider: () => provider,
+			resolveTools: () => [toolA],
+			applyToolsFilter: transformingFilter,
+			runTurn: captureRunTurn,
+		});
+
+		await orchestrator.handleMessage({
+			conversationId: "conv-filter-once",
+			text: "hi",
+			onEvent: () => {},
+		});
+
+		expect(filterCallCount).toBe(1);
+		expect(captured).toHaveLength(1);
+		expect(captured[0]?.tools).toHaveLength(1);
+		expect(captured[0]?.tools[0]?.name).toBe("tool-b");
+	});
+
+	it("tools filter identity is a no-op (same tools reach runTurn)", async () => {
+		const store = createInMemoryStore();
+		const provider: ProviderContract = { id: "p", stream: async function* () {} };
+		const { captured, captureRunTurn } = createCapturingRunTurn();
+
+		const toolA = createFakeTool("tool-a", async () => ({ content: "a" }));
+		const toolB = createFakeTool("tool-b", async () => ({ content: "b" }));
+
+		const orchestrator = createSessionOrchestrator({
+			conversationStore: store,
+			resolveProvider: () => provider,
+			resolveTools: () => [toolA, toolB],
+			applyToolsFilter: identityApplyToolsFilter,
+			runTurn: captureRunTurn,
+		});
+
+		await orchestrator.handleMessage({
+			conversationId: "conv-filter-identity",
+			text: "hi",
+			onEvent: () => {},
+		});
+
+		expect(captured).toHaveLength(1);
+		expect(captured[0]?.tools).toHaveLength(2);
+		expect(captured[0]?.tools[0]?.name).toBe("tool-a");
+		expect(captured[0]?.tools[1]?.name).toBe("tool-b");
+	});
+
+	it("threads cwd and conversationId into the tool assembly", async () => {
+		const store = createInMemoryStore();
+		const provider: ProviderContract = { id: "p", stream: async function* () {} };
+		const { captureRunTurn } = createCapturingRunTurn();
+
+		let receivedAssembly: ToolAssembly | undefined;
+		const capturingFilter = (assembly: ToolAssembly): Promise<ToolAssembly> => {
+			receivedAssembly = assembly;
+			return Promise.resolve(assembly);
+		};
+
+		const orchestrator = createSessionOrchestrator({
+			conversationStore: store,
+			resolveProvider: () => provider,
+			resolveTools: () => [],
+			applyToolsFilter: capturingFilter,
+			runTurn: captureRunTurn,
+		});
+
+		await orchestrator.handleMessage({
+			conversationId: "conv-filter-threads",
+			text: "hi",
+			onEvent: () => {},
+			cwd: "/test/dir",
+		});
+
+		expect(receivedAssembly).toBeDefined();
+		expect(receivedAssembly?.conversationId).toBe("conv-filter-threads");
+		expect(receivedAssembly?.cwd).toBe("/test/dir");
+		expect(receivedAssembly?.tools).toEqual([]);
+	});
+});
+
 function createCounterNow(): { now: () => number; tick: (ms: number) => void } {
 	let t = 0;
 	return {
diff --git a/packages/session-orchestrator/src/orchestrator.ts b/packages/session-orchestrator/src/orchestrator.ts
index d84b805..2d1bbf5 100644
--- a/packages/session-orchestrator/src/orchestrator.ts
+++ b/packages/session-orchestrator/src/orchestrator.ts
@@ -13,6 +13,7 @@ import type {
 import { defineService } from "@dispatch/kernel";
 import { createMetricsAccumulator } from "./metrics.js";
 import { buildUserMessage, defaultDispatchPolicy, generateTurnId } from "./pure.js";
+import type { ToolAssembly } from "./tools-filter.js";
 
 export interface SessionOrchestrator {
 	handleMessage(input: {
@@ -38,6 +39,8 @@ export interface SessionOrchestratorDeps {
 		modelName: string,
 	) => { provider: ProviderContract; model: string } | undefined;
 	readonly runTurn: (input: RunTurnInput) => Promise<RunTurnResult>;
+	/** Apply the per-turn tools filter chain. Injected for testability. */
+	readonly applyToolsFilter: (assembly: ToolAssembly) => Promise<ToolAssembly>;
 	/** Base logger (auto-scoped to this extension); childed per turn for span capture. */
 	readonly logger?: Logger;
 	/** Injected monotonic-ish clock (ms) forwarded to RunTurnInput for timing events. */
@@ -71,7 +74,12 @@ export function createSessionOrchestrator(deps: SessionOrchestratorDeps): Sessio
 				provider = deps.resolveProvider();
 			}
 
-			const tools = deps.resolveTools();
+			const baseTools = deps.resolveTools();
+			const assembled = await deps.applyToolsFilter({
+				tools: baseTools,
+				conversationId,
+				...(cwd !== undefined ? { cwd } : {}),
+			});
 			const dispatch = deps.resolveDispatch?.() ?? defaultDispatchPolicy();
 			const turnLogger = deps.logger?.child({ conversationId, turnId });
 			const metrics = createMetricsAccumulator();
@@ -84,7 +92,7 @@ export function createSessionOrchestrator(deps: SessionOrchestratorDeps): Sessio
 			const opts: RunTurnInput = {
 				provider,
 				messages: [...history, userMsg],
-				tools,
+				tools: assembled.tools,
 				dispatch,
 				emit: emitAndAccumulate,
 				conversationId,
diff --git a/packages/session-orchestrator/src/tools-filter.ts b/packages/session-orchestrator/src/tools-filter.ts
new file mode 100644
index 0000000..19b2eb3
--- /dev/null
+++ b/packages/session-orchestrator/src/tools-filter.ts
@@ -0,0 +1,16 @@
+import { defineFilter, type FilterDescriptor, type ToolContract } from "@dispatch/kernel";
+
+/** Per-turn tool-assembly value threaded through the `tools` filter chain. */
+export interface ToolAssembly {
+	/** The tool set resolved for this turn (the value filters transform). */
+	readonly tools: readonly ToolContract[];
+	/** This turn's working directory (verbatim from the request), for cwd-aware filters. */
+	readonly cwd?: string;
+	/** The conversation this turn belongs to. */
+	readonly conversationId: string;
+}
+
+/** Filter chain run once per turn to transform the tool set before it reaches runTurn. */
+export const toolsFilter: FilterDescriptor<ToolAssembly> = defineFilter<ToolAssembly>(
+	"session-orchestrator/tools",
+);