fawltydeps ignore_unused names only declared dependencies

## Summary

Drop `vale` from `[tool.fawltydeps] ignore_unused`, leaving only entries
that name a dependency the project actually declares.

## Motivation

Surfaced while auditing the Poetry dependency groups behind the Renovate
pinning rule for the test group. `pyproject.toml:25` reads:

```toml
ignore_unused = ["coveralls", "pytest-cov", "vale"]
```

`coveralls` and `pytest-cov` are declared under
`[tool.poetry.group.test.dependencies]` and imported by nothing, which is
exactly what the setting exists for.

`vale` is not declared in any dependency table and does not appear in
`poetry.lock`. Vale is a Go binary: `.devcontainer/post-create.sh:13-19`
downloads the release tarball from `errata-ai/vale`, and
`.pre-commit-config.yaml:61-64` runs it through the upstream pre-commit
repo. There is no Python package for FawltyDeps to consider unused, so
the entry suppresses a warning that cannot fire.

Nothing is broken today, since FawltyDeps ignores names it does not
recognise. The cost is that the line asserts a Python dependency on Vale
that does not exist, which is the opposite of what a reader needs to know
when working out where the linter comes from.

## Scope

- Remove `"vale"` from `ignore_unused` in `pyproject.toml:25`.

## Acceptance criteria

- [ ] `ignore_unused` names only dependencies declared in a
`pyproject.toml` dependency table
- [ ] `pre-commit run --all-files` passes, including both the
FawltyDeps-undeclared and FawltyDeps-unused hooks