chore(claude-md): correct vale claim and Sphinx extension list

Three follow-up Copilot findings on the post-flip CLAUDE.md:

1. Docker image doesn't bundle vale — the repo has no `.vale.ini`,
   `docker/Dockerfile` doesn't install vale, and there's no Vale CI
   workflow. Replace the entire "Lint (vale)" section with an honest
   description of how linting actually happens (`vyoslinter` /
   `doc-linter.py` in `vyos/.github`, run by `lint-doc.yml` on
   changed files only). The Docker build header is updated too —
   it says "bundles Sphinx and the MyST/RTD plugin set" instead of
   the inaccurate "incl. vale".

2. `docs/_ext/releasenotes.py` and `testcoverage.py` are not Sphinx
   extensions — neither has a `setup()` function and neither is in
   `extensions = [...]` in `conf.py`. Reword the bullet so the only
   files described as Sphinx extensions are `vyos.py` and
   `autosectionlabel.py` (both registered in `extensions`); the other
   two are described as standalone helper scripts.

Addresses Copilot review feedback on PR #1902.

\xf0\x9f\xa4\x96 Generated by [robots](https://vyos.io)

0 files changed, 0 insertions, 0 deletions