Add resume_agent collab tool (#10903)

Summary
- add the new resume_agent collab tool path through core, protocol, and
the app server API, including the resume events
- update the schema/TypeScript definitions plus docs so resume_agent
appears in generated artifacts and README
- note that resumed agents rehydrate rollout history without overwriting
their base instructions

Testing
- Not run (not requested)

codex-rs/app-server-protocol/schema/json/v2/ThreadForkResponse.json

@@ -207,6 +207,7 @@
       "enum": [
         "spawnAgent",
         "sendInput",
+        "resumeAgent",
         "wait",
         "closeAgent"
       ],