Fix stale cache refresh for updated session files

[AliFozooni]

Summary

• derive the cached folder index timestamp from the project directory and its root session .jsonl files
• use that timestamp consistently in the initial worker scan and in background refresh checks
• add a regression test for appending to an existing session file

Why

Background refresh currently keys off the project directory mtime. Appending to an existing session file updates the file mtime but usually leaves the directory mtime unchanged, so long-running sessions can keep stale ordering, message counts, titles, and search data until a new file appears or the app restarts.

Verification

• npm test