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/schema/src/session-event.ts

@@ -138,6 +138,7 @@ export const Synthetic = Event.define({
     messageID: SessionMessage.ID,
     text: Schema.String,
     description: Schema.String.pipe(optional),
+    metadata: Schema.Record(Schema.String, Schema.Unknown).pipe(optional),
   },
 })
 export type Synthetic = typeof Synthetic.Type