Quote the coverage `--omit` pattern and drop the dead `.tox/*` from it

This repo has been migrated to uv, so there is no `tox.ini` any more — but a few places still refer to tox. Nothing is broken; it is misleading for the next person (and for agents) reading the code.

Leftovers on `main`:
- `.github/workflows/coverage.yml:25`
```
uv run --group coverage coverage html --omit .tox/*,unittests/*
```
- `.github/workflows/coverage.yml:26`
```
uv run --group coverage coverage report --fail-under 92 --omit .tox/*,unittests/*
```

**To do**
- [ ] drop the dead `.tox/*` pattern **and quote the remaining pattern** in both invocations: `uv run --group coverage coverage html --omit "unittests/*"` and `uv run --group coverage coverage report --fail-under 92 --omit "unittests/*"`. The quoting is the important half — the argument is currently unquoted, and today `.tox/*,unittests/*` survives as one literal word that matches nothing. Remove `.tox/*` without quoting and bash expands `unittests/*` into `unittests/__init__.py unittests/conftest.py …`, so everything after the first file becomes a *positional* argument and `coverage report` silently measures only those files — i.e. the `--fail-under 92` gate stops meaning what it says.
- [ ] no need to add `.venv/*`: coverage.py skips site-packages by default and this repo has no `[tool.coverage]`/`.coveragerc`. If you want the belt-and-braces form anyway, `decidalo_client.py` uses `--omit ".venv/*,unittests/*"`.

Found by an org-wide sweep for `tox` mentions in repos that no longer have a `tox.ini`. Deliberately excluded: historical plan/spec records under `docs/**/plans/**` and `docs/**/specs/**` (a frozen record) and the `.tox/` line in `.gitignore` (harmless).