feat: add file-based logging and replace operation_id with commit_id
- Add Logs library with file reporter (~/.cache/lens/lens.log)
- Replace Lwt_process.pread with run_jj helper that captures stderr
- Log jj subprocess calls at debug level, errors on failure
- Add logging to irmin store operations
- Replace operation_id with commit_id in approval tracking
- Add error handling (Lwt.catch) around store reads and app startup