fix: improve storage reliability with better error handling and lock cleanup

[zenyr]

Summary

Improve storage system reliability with better error handling and lock cleanup.

Changes

JSON Error Handling

• Corrupted JSON files in migration are now skipped with warnings instead of failing
• Prevents migration from crashing on individual corrupted files

Lock Cleanup

• Automatic cleanup for unused locks after 10 seconds of inactivity
• Prevents memory leaks in long-running processes
• Periodic cleanup every 5 seconds

Global Lock Maintained

• Storage operations require global synchronization due to interdependencies
• Ensures data consistency across session/message/part files

Notes

During testing, concurrent session creation showed JSON parsing errors in session.prompt that may indicate issues with repeated session reads during doom loop detection. Further investigation recommended.