Build .github/workflows/ CI pipeline (Vale, markdownlint, mint validate/broken-links)

[digitaldiina]

This repo currently has no .github/workflows/ directory at all — Vale (#82), markdownlint, and mint validate all run locally/manually today, never in CI.

Split out from #82 once Vale's own config landed there and it became clear "wire it into the same CI pass as the Markdownlint step" assumes a CI pass that doesn't exist yet.

Scope: build the actual GitHub Actions workflow(s) for this repo's per-PR CI — fast, incremental checks only:

• Run vale sync && vale <changed files> (config already in .vale.ini, non-blocking per MinAlertLevel = suggestion)
• Run markdownlint-cli against changed .md/.mdx files (config in .markdownlint.yaml/.markdownlintignore)
• Run mint validate against changed files (note: this environment has hit a hard Windows EMFILE: too many open files ceiling running mint validate locally on large PRs — worth checking whether that recurs in a Linux GitHub Actions runner, or whether a raised file-descriptor limit / different validation strategy is needed there too)
• Decide blocking vs advisory per check (Vale/markdownlint likely advisory-only at first per Adapt lint tooling for Mintlify: Markdownlint + Vale #82's own decision; mint validate parse errors probably should block)

Explicitly out of scope: mint broken-links (repo-wide broken-link checking). A full-repo broken-link scan is too slow to run as a per-PR gate — that's tracked separately as a periodic/scheduled check in #81, not part of this workflow.

Milestone: CI/CD pipeline (owner: Bergfrid).