Docs quality gates in CI: link check, spelling and style #66

Why

mkdocs build --strict catches broken internal links and nothing else. External links rot, typos slip in, and with a course landing in docs/learn/ the volume of prose is about to triple.

What

• lychee for external links on PRs touching docs/** or README.md (cache, exclude localhost and the demo URLs, fail on 404/410, warn on timeouts).
• codespell over docs/, README.md, AGENTS.md, CONTRIBUTING.md, SECURITY.md and Go comments, with an ignore list for identifiers (apic, httpbin, gjson, sigv4, …).
• Vale with the write-good or Google style base and a small project vocabulary (docs/.vale/), tuned so the existing pages pass; British spelling, since the project already writes "licence" and "colour".
• task docs:check runs all three locally; docs.yml runs them before the build.

Where

.github/workflows/docs.yml, Taskfile.yml, .lychee.toml, .codespellrc, .vale.ini, docs/.vale/, CONTRIBUTING.md.

Tests

The workflow itself; a deliberate typo in a PR must fail.