feat(core): tool execute hooks, session instructions, synthetic endpoint

- V2 tool execute hooks: add `ctx.tool.execute.before/after` plugin API,
  core ToolHooks service, and registry wiring so hosted/local tool calls
  run registered before/after hooks; provider-executed calls are excluded.
- Session synthetic endpoint: add `POST /api/session/:sessionID/synthetic`
  with text/description/metadata plus regenerated Promise/Effect/JS client
  surfaces.
- SessionInstructions service: read tool discovers nearby AGENTS.md walking
  up to the Location root (exclusive) and injects them as durable synthetic
  instructions, with lazy history dedup of prior claims.
- to-llm-message: stop forwarding synthetic message metadata to the model so
  bookkeeping annotations stay model-hidden.
- schema changelog entry for synthetic metadata and the metadata leak fix.

packages/core/test/plugin/host.ts

@@ -52,6 +52,10 @@ export function host(overrides: Overrides = {}): PluginContext {
     },
     tool: overrides.tool ?? {
       register: () => Effect.die("unused tool.register"),
+      execute: {
+        before: () => Effect.die("unused tool.execute.before"),
+        after: () => Effect.die("unused tool.execute.after"),
+      },
     },
     session: overrides.session ?? {
       create: () => Effect.die("unused session.create"),