Describe the Bug

Editing or deleting a message can corrupt a thread's messages.jsonl when the app/process is interrupted mid-write.

Current flow rewrites the entire file using truncate-first semantics. If interruption happens between truncate and full rewrite completion, the file can end up partially written. On next read, one malformed/truncated line causes full read failure for that thread.

This is a critical stability issue because it can make existing chat threads unreadable after a crash/power loss/forced close during normal operations (modify_message / delete_message).

Steps to Reproduce

1. Create a thread with multiple messages.
2. Trigger modify_message or delete_message for that thread.
3. Force-close/kill app process during rewrite window.
4. Re-open app and load the thread.
5. Observe message loading failure for the whole thread.

Operating System

• MacOS
• Windows
• Linux