[codex-backend] Prefer sqlite git info for rollout-path reads

[joeytrasatti-openai]

Summary

• Path-based local thread reads currently return rollout/session git metadata directly, so thread/resume can disagree with persisted SQLite metadata for the same thread.
• Merge non-null SQLite git fields over rollout-path reads while keeping rollout values as fallbacks for fields SQLite does not know.
• Add focused regression coverage for rollout-path reads so persisted branch updates are preserved during resume.

Testing

• cargo test -p codex-thread-store