feat: fork conversation/thread (#8866)

## Summary
- add thread/conversation fork endpoints to the protocol (v1 + v2)
- implement fork handling in app-server using thread manager and config
overrides
- add fork coverage in app-server tests and document `thread/fork` usage

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -5,6 +5,7 @@ mod output_schema;
 mod rate_limits;
 mod review;
 mod thread_archive;
+mod thread_fork;
 mod thread_list;
 mod thread_loaded_list;
 mod thread_resume;