ci: add demo validation workflow

[DanFashauer]

Summary

• Adds .github/workflows/demo-validation.yml to validate demo-related changes on pull requests and pushes to main.
• Scopes the workflow to demo/API/demo-doc/package/workflow paths.
• Runs deterministic validation: npm ci, typecheck, lint, build, the demo session-start Vitest test, and npm run demo:media.
• Uploads artifacts/demo-media/ as a workflow artifact and treats storyboard fallback as valid when capture-report.json is produced.
• Uses local-only demo validation environment variables and does not require production secrets.

Files changed

• .github/workflows/demo-validation.yml

Validation

• Recreated through the GitHub connector because the Codex workspace did not return a GitHub-visible PR URL.
• No product logic changed.