Every message called _persist(), which rewrote the ENTIRE
desktop_sessions.json (all sessions, full messages + llm_history) under
the lock. Write cost was O(all sessions), so performance degraded badly
as sessions piled up.
Now each session is its own temp/desktop_sessions/<id>.json:
- _persist_session(s): atomic per-session write, O(one session)
- _delete_session_file(sid): remove on delete
- add_message / create / plan-bind / set_model / patch now write only
the touched session; write cost no longer scales with session count
- _load_sessions reads the dir; one-time migration from the legacy
monolithic file, which is then renamed to .json.migrated
- import_sessions reads both source formats (per-file dir + legacy),
persists only newly added sessions
yiqi-017
bdd5309607
