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/runner/to-llm-message.ts

@@ -130,7 +130,7 @@ function toLLMMessage(message: SessionMessage.Message, model: Model): Message[]
         }),
       ]
     case "synthetic":
-      return [Message.make({ id: message.id, role: "user", content: message.text, metadata: message.metadata })]
+      return [Message.make({ id: message.id, role: "user", content: message.text })]
     case "skill":
       return [Message.make({ id: message.id, role: "user", content: message.text, metadata: message.metadata })]
     case "system":