chore: exclude internal implementation docs from repository

Clean up repository by excluding internal planning and implementation
documents that are not relevant for end users.

Changes:
- Added documentation/implementation/* to .gitignore
- Removed SKILLS_REFACTORING_PLAN.md from git tracking
- File remains locally for maintainer use

Excluded Documents:
- documentation/implementation/SKILLS_REFACTORING_PLAN.md (internal planning)
- Future implementation docs in documentation/implementation/

Kept Documents (User-Facing):
- All root .md files (README, CONTRIBUTING, CHANGELOG, etc.)
- documentation/PYTHON_TOOLS_AUDIT.md (transparency about tool quality)
- documentation/GIST_CONTENT.md (excluded but committed initially)

Rationale:
- Root files follow open source best practices
- Python tools audit provides transparency
- Implementation planning is internal-only
- Cleaner repository for users
- Maintains professional appearance

All user-facing documentation remains accessible and comprehensive.

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

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -15,4 +15,5 @@ PROMPTS.md
 .env.test.local
 .env.production.local
 medium-content-pro/*
-documentation/GIST_CONTENT.md
+documentation/GIST_CONTENT.md
+documentation/implementation/*