fix(session): handle errors in legacy session migration

shutil.move() in _load() can fail due to permissions, disk full, or
concurrent access. Without error handling, the exception propagates up
and prevents the session from loading entirely.

Wrap in try/except so migration failures are logged as warnings and the
session falls back to loading from the legacy path on next attempt.

Fixes #863

nanobot/session/manager.py

@@ -108,9 +108,12 @@ class SessionManager:
         if not path.exists():
             legacy_path = self._get_legacy_session_path(key)
             if legacy_path.exists():
-                import shutil
-                shutil.move(str(legacy_path), str(path))
-                logger.info("Migrated session {} from legacy path", key)
+                try:
+                    import shutil
+                    shutil.move(str(legacy_path), str(path))
+                    logger.info("Migrated session {} from legacy path", key)
+                except Exception as e:
+                    logger.warning("Failed to migrate session {}: {}", key, e)
 
         if not path.exists():
             return None