Set up GitHub Actions automation

[engmung]

Automate routine repo tasks with GitHub Actions (.github/workflows/). Candidate workflows, roughly in priority order:

• Web CI — on PRs touching web/**, run next build + lint so broken code can't reach main.
• Roadmap snapshot — on a schedule (and on issue events), fetch issues / project data and commit web/public/roadmap.json. The /roadmap page then reads a static file with no runtime token and no rate limit, replacing the current live /api/roadmap fetch.
• Discord notifications — post to a Discord webhook on release published / issue opened.
• Auto-labeling / triage — label PRs by changed paths (area:web / area:firmware / …); optional stale-bot for old issues.

Notes:

• Public repo → free unlimited Actions minutes.
• Secrets (tokens, webhook URLs) live in Settings → Secrets → Actions, referenced as ${{ secrets.NAME }}.