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.

1 files changed, 2 insertions, 0 deletions

diff --git a/packages/host-bin/src/main.ts b/packages/host-bin/src/main.ts
index 588dfb8..ef75f55 100644
--- a/packages/host-bin/src/main.ts
+++ b/packages/host-bin/src/main.ts
@@ -20,6 +20,7 @@ import {
 } from "@dispatch/kernel";
 import { extension as providerOpenaiCompatExt } from "@dispatch/provider-openai-compat";
 import { extension as sessionOrchestratorExt } from "@dispatch/session-orchestrator";
+import { extension as skillsExt } from "@dispatch/skills";
 import { createSqliteStorage, extension as storageSqliteExt } from "@dispatch/storage-sqlite";
 import { createLoadedExtensionsExtension } from "@dispatch/surface-loaded-extensions";
 import { createSurfaceRegistryExtension } from "@dispatch/surface-registry";
@@ -71,6 +72,7 @@ const CORE_EXTENSIONS: readonly Extension[] = [
 	toolWriteFileExt,
 	throughputStoreExt,
 	sessionOrchestratorExt,
+	skillsExt,
 	createTransportHttpExtension(),
 	// Surface extensions — dependency order: surface-registry first, then consumers.
 	createSurfaceRegistryExtension(),