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, 20 insertions, 0 deletions

diff --git a/packages/kernel/src/contracts/extension.ts b/packages/kernel/src/contracts/extension.ts
index 1760cf9..5a7821b 100644
--- a/packages/kernel/src/contracts/extension.ts
+++ b/packages/kernel/src/contracts/extension.ts
@@ -196,6 +196,26 @@ export interface HostAPI {
 		fn: FilterHandler<TValue>,
 	) => () => void;
 
+	/**
+	 * Run a filter chain: thread `value` through every filter registered for
+	 * `hook` in priority/registration order and return the final value. The
+	 * single-value-in/value-out counterpart to `addFilter`. Awaited in-band.
+	 *
+	 * Fail-open by default (a thrown filter is logged and the value passes
+	 * through unchanged); pass `{ failClosed: true }` to make a thrown filter
+	 * reject. With no registered filters the input value is returned as-is.
+	 *
+	 * This is what lets a core extension expose a contribution point (e.g. the
+	 * session-orchestrator running a per-turn tool/context-assembly chain) that
+	 * standard extensions plug into via `addFilter` — the kernel owns the
+	 * mechanism, the owner declares the typed `FilterDescriptor`.
+	 */
+	readonly applyFilters: <TValue>(
+		hook: FilterDescriptor<TValue>,
+		value: TValue,
+		opts?: { readonly failClosed?: boolean },
+	) => Promise<TValue>;
+
 	/** Provide an implementation for a typed service handle. */
 	readonly provideService: <T>(handle: ServiceHandle<T>, impl: T) => void;