`test_chat_oneshot_success` proved that an explicit `[database].url` wins over
the durable default by stat'ing the *real* `~/.local/share/plyngent/chat.db` and
comparing its mtime. So a test read the user's own session database, and it went
red whenever a live chat happened to write it — which is what a run does while
another plyngent session is open.
The default path is now pointed into `tmp_path`, the way `test_app.py` already
does it, and the assertion is that no `chat.db` shows up there. Nothing outside
tmp_path is read, and a regression — the explicit url ignored — still fails,
because the default branch would create that file.
NCBM