refactor(skills): rename custom skills to follow gerund naming conventions
Per PR review feedback from Eric, renamed all custom skill directories
to use gerund form following Skills naming best practices:
- brand-guidelines → applying-brand-guidelines
- financial-analyzer → analyzing-financial-statements
- financial-modeler → creating-financial-models
Additional fixes from PR review:
- Fix capabilities/README.md title from "Claude Skills" to "Claude Capabilities"
- Remove internal planning document (skills_cookbook_plan.md)
- Remove status line from skills/CLAUDE.md
- Update SDK references from local whl to PyPI package (>=0.71.0)
- Fix repository URLs from private to public (claude-cookbooks)
- Remove absolute file paths from documentation
- Update all notebook references to use new skill directory names
- Fix broken link in contextual-embeddings notebook
- Update SKILL.md descriptions to use third person
- Remove references to non-existent Python scripts in financial-modeler
This ensures the cookbook follows Anthropic's Skills naming conventions
and is ready for public release.