Document the formatting and lint gates (treefmt/nixfmt/shfmt/prettier,
deadnix, statix, pre-commit) and how to run them, so changes -- docs
included -- are formatted before commit. Notes the CI detect step that
skips heavy checks on docs-only PRs, which can report a false green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
