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/protocol/src/groups/session.ts

@@ -277,6 +277,26 @@ export const makeSessionGroup = <I extends HttpApiMiddleware.AnyId, S>(sessionLo
           }),
         ),
     )
+    .add(
+      HttpApiEndpoint.post("session.synthetic", "/api/session/:sessionID/synthetic", {
+        params: { sessionID: Session.ID },
+        payload: Schema.Struct({
+          text: Schema.String,
+          description: Schema.String.pipe(Schema.optional),
+          metadata: SessionMessage.Synthetic.fields.metadata,
+        }),
+        success: HttpApiSchema.NoContent,
+        error: SessionNotFoundError,
+      })
+        .middleware(sessionLocationMiddleware)
+        .annotateMerge(
+          OpenApi.annotations({
+            identifier: "v2.session.synthetic",
+            summary: "Add synthetic message",
+            description: "Append a synthetic message to a session and resume execution.",
+          }),
+        ),
+    )
     .add(
       HttpApiEndpoint.post("session.compact", "/api/session/:sessionID/compact", {
         params: { sessionID: Session.ID },