chore: move SQL and schema development docs to Claude skills (#296)

# Reorganize SQL and Schema Development Documentation into Skills

This PR reorganizes the SQL and schema development documentation into dedicated skills for better organization and accessibility. The changes include:

- Created a new `schema-dev` skill with comprehensive guidance on SQL schema development workflow, TDD approach, and troubleshooting
- Added SQL style guidelines as a reference document within the schema-dev skill
- Created a new `migration-management` skill with detailed instructions for generating, regenerating, and handling migrations
- Removed standalone SQL style and schema development documents from the core and reference directories
- Updated the CLAUDE.md file to remove the reference to the removed SQL style document

These changes make the documentation more structured and easier to find when needed, while preserving all the essential guidance for SQL development and migration management.

Author: jumski