feat: implement conditional rule discovery engine

[dmoliveira]

Summary

• Add scripts/rules_engine.py with markdown frontmatter parsing, layered rule discovery (user/project), path-based applicability matching, deterministic precedence ordering, and duplicate-id conflict reporting.
• Expand selftest coverage to validate frontmatter parsing, layered discovery, precedence conflict behavior, and target-path rule resolution.
• Mark Epic 9 Task 9.2 complete in roadmap and record the engine/test additions in changelog.

Testing

• make validate
• make selftest
• make install-test