SSH: symlink-staging + tar -ch piped over SSH in a single TCP stream.
Eliminates per-file scp round-trips. Handles timeout (kills both
processes), SSH Popen failure (kills tar), and tar create failure.
Modal: in-memory gzipped tar archive, base64-encoded, decoded+extracted
in one exec call. Checks exit code and raises on failure.
Both backends use shared helpers extracted into file_sync.py:
- quoted_mkdir_command() — mirrors existing quoted_rm_command()
- unique_parent_dirs() — deduplicates parent dirs from file pairs
Migrates _ensure_remote_dirs to use the new helpers.
28 new tests (21 SSH + 7 Modal), all passing.
Closes#7465Closes#7467
