Add definition-of-done and pull-request-compliance actions

[yanjost]

Summary

This draft introduces two new reusable GitHub actions to improve PR governance:

1. definition-of-done

• Validates the PR body for a Definition of Done section.
• Fails if the section exists with no checklist items.
• Fails if any checklist item is unchecked.
• Supports configurable heading text via section-heading.

2. pull-request-compliance

• Runs deterministic checks on PR title/body:
  • non-empty description
  • Definition of Done checklist enforcement
  • forbidden placeholders (TBD, TODO, ??)
• Optionally runs OpenAI-based policy/template validation when openai-api-key is provided.
• Gracefully skips LLM validation with a warning when no API key is configured.

Additional changes

• Added docs and local usage examples under each action:
  • definition-of-done/examples/workflow.yml
  • pull-request-compliance/examples/workflow.yml
• Added unit tests for both actions:
  • definition-of-done/tests/test_check_definition_of_done.py
  • pull-request-compliance/tests/test_check_pr_compliance.py
• Added CI jobs to run both test suites.
• Updated root README with a new Pull Request Governance section.

Verification

• ./definition-of-done/tests/run.sh
• ./pull-request-compliance/tests/run.sh