- Restore null CWD in sessionOverrides for thread/start and thread/resume
  so the Codex app server can distinguish 'no override' from an explicit
  path (e.g., to reuse a resumed thread's original CWD).
- Use backreference in diff --git fallback regex to correctly parse paths
  containing ' b/' (e.g., binary diffs without +++ b/ lines).
- Add test exercising the git header fallback for binary diffs with ' b/'
  in the path.

Applied via @cursor push command