Add sync_back() to FileSyncManager — on sandbox cleanup, downloads
the remote .hermes/ directory as a tar archive, diffs against SHA-256
hashes of what was originally pushed, and applies only changed files.
- SHA-256 content hashing on push for accurate change detection
- Retry with exponential backoff (3 attempts, 2s/4s/8s)
- SIGINT deferred during sync-back to prevent partial writes
- fcntl.flock serialization for concurrent gateway sandboxes
- Last-write-wins conflict resolution with logged warnings
- New files created on remote are pulled back via path inference
- Backend implementations: SSH (tar cf over pipe), Modal (exec tar
cf, read stdout), Daytona (exec tar cf, SDK download_file)
- Wired into cleanup() for all three backends (runs before
ControlMaster close / sandbox terminate / sandbox stop)
28 new tests (10 FSM core + 18 backend-specific), 72 total passing.
