Replace per-backend ad-hoc file sync with a shared FileSyncManager
that handles mtime-based change detection, remote deletion of
locally-removed files, and transactional state updates.
- New FileSyncManager class (tools/environments/file_sync.py)
with callbacks for upload/delete, rate limiting, and rollback
- Shared iter_sync_files() eliminates 3 duplicate implementations
- SSH: replace unconditional rsync with scp + mtime skip
- Modal/Daytona: replace inline _synced_files dict with manager
- All 3 backends now sync credentials + skills + cache uniformly
- Remote deletion: files removed locally are cleaned from remote
- HERMES_FORCE_FILE_SYNC=1 env var for debugging
- Base class _before_execute() simplified to empty hook
- 12 unit tests covering mtime skip, deletion, rollback, rate limiting
