ci: enhance PR lint and add PR guidelines to CONTRIBUTING.md

[vividctrlalt]

Summary

• Enhance the PR lint job in pr-check.yml with stricter validation rules aligned with CONTRIBUTING.md:
  • Merge title and description checks into a single step
  • Add no trailing period check
  • Add no capitalized description check
  • Add known scope validation (warning only, not blocking)
  • Add CONTRIBUTING.md doc link in error messages
  • Improve regex to use [^)]+ for scope matching
• Add comprehensive PR guidelines to CONTRIBUTING.md:
  • PR Title Format rules (conventional commit format, length limits)
  • Type and Scope Reference tables (module, functional domain, feature scopes)
  • PR Description requirements
  • General Rules for PR submission

Test plan

• Submit a PR with title ending in . → should fail
• Submit a PR with capitalized description → should fail
• Submit a PR with unknown scope → should show warning (not fail)
• Submit a PR with valid title and description → should pass
• Verify error messages include CONTRIBUTING.md link