feat: add markdown config risk summaries

[mattpartida]

Summary

• Adds a --format markdown option to config_risk_summary.py while preserving JSON as the default output.
• Renders severity counts and findings in a Markdown table with rule IDs, fields, recommendations, and escaped table cells.
• Documents Markdown output usage in the README.

Test Plan

• python -m compileall -q skills tests
• python -m pytest -q
• ruff check .
• ./package-skills.sh
• git diff --check