fix(file-sync): resolve 4 bugbot findings in sync-back

1. Tar paths now match _pushed_hashes keys — backends tar from /
   so entries have full absolute paths (e.g. root/.hermes/skills/f.py)
   instead of relative ./skills/f.py that never matched hash lookups
2. _infer_host_path simplified — removed broken grandparent match
   that computed garbled suffixes for new remote files
3. Lock path uses get_hermes_home() instead of Path.home() — fixes
   wrong lock path when HERMES_HOME is overridden or using profiles
4. SIGINT trap guarded by threading.current_thread() check — skips
   signal.signal() on non-main threads (gateway workers) instead of
   crashing with ValueError on every retry attempt

tools/environments/modal.py

@@ -353,7 +353,7 @@ class ModalEnvironment(BaseEnvironment):
         """Download remote .hermes/ as a tar archive."""
         async def _download():
             proc = await self._sandbox.exec.aio(
-                "bash", "-c", "tar cf - -C /root/.hermes ."
+                "bash", "-c", "tar cf - -C / root/.hermes"
             )
             data = await proc.stdout.read.aio()
             exit_code = await proc.wait.aio()