Merge PR #621: fix: limit concurrent Modal sandbox creations to avoid deadlocks

Authored by voteblake.

- Semaphore limits concurrent Modal sandbox creations to 8 (configurable)
  to prevent thread pool deadlocks when 86+ tasks fire simultaneously
- Modal cleanup guard for failed init (prevents AttributeError)
- CWD override to /app for TB2 containers
- Add /home/ to host path validation for container backends

tools/environments/modal.py

@@ -137,6 +137,10 @@ class ModalEnvironment(BaseEnvironment):
 
     def cleanup(self):
         """Snapshot the filesystem (if persistent) then stop the sandbox."""
+        # Check if _inner was ever set (init may have failed)
+        if not hasattr(self, '_inner') or self._inner is None:
+            return
+
         if self._persistent:
             try:
                 sandbox = getattr(self._inner, 'deployment', None)