refactor(core): standardize commit timestamp extraction and backup ref parsing

[JacksonFergusonDev]

Summary

This PR resolves DRY violations related to Git commit timestamp extraction, remote tracking reference construction, and backup ref parsing:

1. Standardized Commit Timestamp Extraction:

   • Added GitRepo.get_commit_timestamp(self, ref: str) -> int | None in git_wrapper.py.
   • Replaced scattered git log -1 --format=%ct invocations and ad-hoc parsing across daemon.py, ops.py, and cli.py.

2. Structured Backup Reference Parsing:

   • Introduced BackupRefInfo dataclass and parse_backup_ref(ref: str) -> BackupRefInfo | None in ops.py.
   • Replaced fragile split('/')[-2] and dynamic slice indexing across ops.get_remote_drift_state, ops.sync_session, ops.finalize_work, and system._fetch_remote_identities.

3. Consolidated Remote Backup Ref Formatting & Fetching:

   • Added ops.get_remote_backup_ref(branch: str, remote_name: str = 'origin') -> str.
   • Added ops.fetch_backup_refs(repo: GitRepo, branch: str | None = None, remote_name: str = 'origin') -> bool.
   • Replaced duplicate fetch refspecs and string stripping in daemon.run_backup, cli.show_status, ops.get_remote_drift_state, ops.sync_session, and ops.finalize_work.

4. Automated Tests:

   • Added unit tests for GitRepo.get_commit_timestamp.
   • Added unit tests for parse_backup_ref, get_remote_backup_ref, and fetch_backup_refs.

Verification

• All 162 pytest tests passing locally.
• Pre-commit hooks (ruff check, ruff format, mypy) passing cleanly.