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/src/session.ts

@@ -199,6 +199,7 @@ export interface Interface {
     sessionID: SessionSchema.ID
     text: string
     description?: string
+    metadata?: Record<string, unknown>
   }) => Effect.Effect<void, NotFoundError>
   readonly revert: {
     readonly stage: (input: {
@@ -547,6 +548,7 @@ const layer = Layer.effect(
           timestamp: yield* DateTime.now,
           text: input.text,
           description: input.description,
+          metadata: input.metadata,
         })
         yield* execution.resume(input.sessionID).pipe(Effect.ignore, Effect.forkIn(scope, { startImmediately: true }), Effect.asVoid)
       }),