docs: state that .gitignore is not consulted, with the ignore key as the exclusion list

[MaximeGaudin]

Summary

• .gitignore was never consulted, so a git-ignored build/output directory was linted by default, and nothing said why.
• This issue offered two resolutions: walk with the ignore crate (a new dependency tree the repo does not currently carry), or document the tradeoff. This PR takes the documented-tradeoff option the issue names: the README and the config docs now state that .gitignore is not read and that the ignore key is the exclusion list, with a **/build-output/** example.
• A regression test pins the behaviour — a git-ignored directory is linted — so a future change that starts honouring .gitignore (for example by adopting the ignore crate) cannot land silently; the test and the docs must be updated together.

Fixes #37

Test plan

• New test pins the documented behaviour (passes before and after; it is a contract pin, not a red test, because this resolution is documentation)
• Local checks mirror CI and passed before push (./scripts/check.sh, JS included)
• CI green on the PR