docs: Add SPEC-19 investigation for Claude Skills integration

Comprehensive investigation document covering:
- Integration opportunities between Basic Memory and Agent Skills
- Export/import workflows for skills
- Enhanced MCP resource patterns
- Native skill entity type proposal
- Implementation phases and evaluation criteria

References issue #372

Co-authored-by: Paul Hernandez <phernandez@users.noreply.github.com>

Author: github-actions[bot]