docs: add comprehensive plan for SQL Doxygen documentation

Add detailed implementation plan for adding Doxygen-style comments to all
53 SQL implementation files across the EQL codebase.

Plan includes:
- 6 phases with 29 tasks (21-31 hour estimate)
- Complete file checklist (53 files across 13 modules)
- Validation scripts for coverage, syntax, and required tags
- Templates and standards documentation
- Quality assurance and handoff procedures

Related to SQL documentation generation RFC.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: tobyhede