Make the local packse workflow work again:
```
# In packse:
uv run --extra index --extra serve packse serve --no-hash scenarios &
# In uv:
UV_TEST_INDEX_URL="http://localhost:3141/simple/" ./scripts/scenarios/generate.py
```
Bugs fixed:
* The default scenario pattern didn't match anything.
* The snapshot update test command was wrong since the test
centralization
* Snapshot update failures would not be reported