fix(ci): add PR lint/typecheck gates and unblock workflow lint

[garethpaul]

Summary

This PR hardens PR CI by adding explicit lint and typecheck gates, and fixes an existing workflow-lint blocker in the reviewer workflow.

Changes

• Updated .github/workflows/pr-checks.yml:
• Added npm run lint
• Added npm run typecheck
• Ordered checks as: npm ci → npm run build → npm run lint → npm run typecheck → npm test
• Updated .github/workflows/pull-request-reviewer.yml:
• Replaced if: ${{ false }} with if: ${{ vars.ENABLE_PR_REVIEWER == 'true' }} so it remains disabled by default but passes actionlint.

Why

• Prevent lint/type regressions from slipping through PR checks.
• Ensure type/lint gates run reliably on clean CI runners (after build artifacts exist).
• Remove unrelated workflow-lint noise so workflow checks are trustworthy.

Validation

• npm run lint:workflows passes.