Add Workshop Playwright QA agentic workflow

[Copilot]

Adds a daily agentic workflow that runs Playwright against the locally-built workshop site to catch navigation regressions, readability issues, and accessibility defects before they reach learners.

Workflow behavior

• Build & serve: installs deps (marked, github-slugger, etc.), runs node scripts/build-docs.js, starts http-server on port 4000
• Page discovery: extracts workshop step pages from <details id="..."> anchors in the built HTML
• Three viewports: mobile 375×667, tablet 768×1024, desktop 1280×800

Test coverage

Category	Checks
Navigation	Page load, no JS errors, hash-anchor targets open, internal link clicks resolve
Readability	Body font ≥ 14 px, no horizontal overflow, non-empty <title>, code block overflow on mobile
Accessibility	<img> alt text, accessible names on interactive elements, single <h1>, heading hierarchy, visible focus rings

Issue reporting

• One issue per page with a findings table + screenshots; updates existing open issues via add-comment instead of duplicating
• Issues labeled bug + accessibility, title-prefixed [workshop-playwright-qa], deduplicated by title, expire after 1 day