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

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.

tools/environments/modal.py

@@ -22,6 +22,7 @@ from tools.environments.base import (
     _save_json_store,
 )
 from tools.environments.file_sync import (
+    BulkDownloadFn,
     FileSyncManager,
     iter_sync_files,
     quoted_mkdir_command,
@@ -269,6 +270,7 @@ class ModalEnvironment(BaseEnvironment):
             upload_fn=self._modal_upload,
             delete_fn=self._modal_delete,
             bulk_upload_fn=self._modal_bulk_upload,
+            bulk_download_fn=self._modal_bulk_download,
         )
         self._sync_manager.sync(force=True)
         self.init_session()
@@ -347,6 +349,23 @@ class ModalEnvironment(BaseEnvironment):
 
         self._worker.run_coroutine(_bulk(), timeout=120)
 
+    def _modal_bulk_download(self, dest: Path) -> None:
+        """Download remote .hermes/ as a tar archive."""
+        async def _download():
+            proc = await self._sandbox.exec.aio(
+                "bash", "-c", "tar cf - -C /root/.hermes ."
+            )
+            data = await proc.stdout.read.aio()
+            exit_code = await proc.wait.aio()
+            if exit_code != 0:
+                raise RuntimeError(f"Modal bulk download failed (exit {exit_code})")
+            return data
+
+        tar_bytes = self._worker.run_coroutine(_download(), timeout=120)
+        if isinstance(tar_bytes, str):
+            tar_bytes = tar_bytes.encode()
+        dest.write_bytes(tar_bytes)
+
     def _modal_delete(self, remote_paths: list[str]) -> None:
         """Batch-delete remote files via exec."""
         rm_cmd = quoted_rm_command(remote_paths)
@@ -404,6 +423,10 @@ class ModalEnvironment(BaseEnvironment):
         if self._sandbox is None:
             return
 
+        if self._sync_manager:
+            logger.info("Modal: syncing files from sandbox...")
+            self._sync_manager.sync_back()
+
         if self._persistent:
             try:
                 async def _snapshot():