[codex-backend] Prefer state git metadata in filtered thread lists

[joeytrasatti-openai]

Summary

• thread/list filtered filesystem results already overlay state DB metadata, but the existing merge only filled missing git fields.
• Prefer non-null SQLite git metadata over stale non-null rollout values so persisted branch/SHA/origin updates are reflected in filtered thread lists.
• Update the focused merge test to cover stale filesystem git metadata being replaced by state-backed values.

Testing

now getting expected icons