[Test Infra] Tighten PR template + add CI enforcement for tests/docs #965

Parent epic: #773

Summary

Add machine-checkable CI gate so PRs cannot merge with unchecked Submission Checklist boxes that lack N/A: <reason> justification, and so the test coverage matrix stays in sync with feature changes.

Scope

• Tighten .github/PULL_REQUEST_TEMPLATE.md — every checkbox needs either [x] or [ ] N/A: <reason>. Add line for "coverage matrix updated" and "happy + failure/edge case covered".
• Cross-link docs/TESTING-STRATEGY.md and docs/TEST-COVERAGE-MATRIX.md from the template.
• New workflow .github/workflows/pr-quality.yml:
  • check-coverage-matrix — parses matrix, fails if any feature ID 0.1.1..13.5.3 missing (uses scripts/feature-ids.json).
  • pr-checklist-guard — parses PR body via gh api, fails on unchecked boxes without N/A:.
  • markdown-link-check over docs/*.md.
• Soft-fail (continue-on-error: true) for first 2 weeks; flip to required after.
• Label-based skip for docs / chore PRs.
• scripts/check-coverage-matrix.mjs + scripts/check-pr-checklist.mjs + Vitest unit tests for parser (5 cases: all-checked pass, unchecked-no-N/A fail, unchecked-with-N/A pass, empty body fail, ignore code-block boxes).

Acceptance

• CI fails (soft) on a PR with unchecked checklist + no N/A:.
• CI fails on a PR that adds a feature without updating the matrix.
• Parser unit tests green.
• 2-week soft window documented in PR body.

Out of scope

• Coverage thresholds (delta-coverage) — separate issue.