feat(file-sync): sync remote changes back to host on teardown

Salvage of PR #8018 by @alt-glitch onto current main.

On sandbox teardown, FileSyncManager now downloads the remote .hermes/
directory, diffs against SHA-256 hashes of what was originally pushed,
and applies only changed files back to the host.

Core (tools/environments/file_sync.py):
- sync_back(): orchestrates download -> unpack -> diff -> apply with:
  - Retry with exponential backoff (3 attempts, 2s/4s/8s)
  - SIGINT trap + defer (prevents partial writes on Ctrl-C)
  - fcntl.flock serialization (concurrent gateway sandboxes)
  - Last-write-wins conflict resolution with warning
  - New remote files pulled back via _infer_host_path prefix matching

Backends:
- SSH: _ssh_bulk_download — tar cf - piped over SSH
- Modal: _modal_bulk_download — exec tar cf - -> proc.stdout.read
- Daytona: _daytona_bulk_download — exec tar cf -> SDK download_file
- All three call sync_back() at the top of cleanup()

Fixes applied during salvage (vs original PR #8018):

| # | Issue | Fix |
|---|-------|-----|
| C1 | import fcntl unconditional — crashes Windows | try/except with fallback; _sync_back_locked skips locking when fcntl=None |
| W1 | assert for runtime guard (stripped by -O) | Replaced with proper if/raise RuntimeError |
| W2 | O(n*m) from _get_files_fn() called per file | Cache mapping once at start of _sync_back_impl, pass to resolve/infer |
| W3 | Dead BulkDownloadFn imports in 3 backends | Removed unused imports |
| W4 | Modal hardcodes root/.hermes, no explanation | Added docstring comment explaining Modal always runs as root |
| S1 | SHA-256 computed for new files where pushed_hash=None | Skip hashing when pushed_hash is None (comparison always False) |
| S2 | Daytona /tmp/.hermes_sync.tar never cleaned up | Added rm -f after download (best-effort) |

Tests: 49 passing (17 new: _infer_host_path edge cases, SIGINT
main/worker thread, Windows fcntl=None fallback, Daytona tar cleanup).

Based on #8018 by @alt-glitch.

tools/environments/daytona.py

@@ -134,6 +134,7 @@ class DaytonaEnvironment(BaseEnvironment):
             upload_fn=self._daytona_upload,
             delete_fn=self._daytona_delete,
             bulk_upload_fn=self._daytona_bulk_upload,
+            bulk_download_fn=self._daytona_bulk_download,
         )
         self._sync_manager.sync(force=True)
         self.init_session()
@@ -166,6 +167,19 @@ class DaytonaEnvironment(BaseEnvironment):
         ]
         self._sandbox.fs.upload_files(uploads)
 
+    def _daytona_bulk_download(self, dest: Path) -> None:
+        """Download remote .hermes/ as a tar archive."""
+        rel_base = f"{self._remote_home}/.hermes".lstrip("/")
+        self._sandbox.process.exec(
+            f"tar cf /tmp/.hermes_sync.tar -C / {shlex.quote(rel_base)}"
+        )
+        self._sandbox.fs.download_file("/tmp/.hermes_sync.tar", str(dest))
+        # Clean up remote temp file
+        try:
+            self._sandbox.process.exec("rm -f /tmp/.hermes_sync.tar")
+        except Exception:
+            pass  # best-effort cleanup
+
     def _daytona_delete(self, remote_paths: list[str]) -> None:
         """Batch-delete remote files via SDK exec."""
         self._sandbox.process.exec(quoted_rm_command(remote_paths))
@@ -213,6 +227,10 @@ class DaytonaEnvironment(BaseEnvironment):
         return _ThreadedProcessHandle(exec_fn, cancel_fn=cancel)
 
     def cleanup(self):
+        if self._sync_manager:
+            logger.info("Daytona: syncing files from sandbox...")
+            self._sync_manager.sync_back()
+
         with self._lock:
             if self._sandbox is None:
                 return