docs: Release v1.18.0 documentation

Update all documentation for v1.18.0 release including:
- Add 4 new skills: pdf-creator, claude-md-progressive-disclosurer, promptfoo-evaluation, iOS-APP-developer
- Update marketplace skills count from 23 to 25
- Update version badges and descriptions across README (EN/ZH)
- Update QUICKSTART guides with clearer marketplace install syntax
- Add skill sections with features, use cases, and installation commands
- Update CLAUDE.md with new skill listings and counts
- Update demos index

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -89,11 +89,11 @@ Before submitting, validate your skill:
 5. **Test locally:**
    ```bash
    # Add your fork as marketplace
-   /plugin marketplace add https://github.com/your-username/claude-code-skills
+   claude plugin marketplace add https://github.com/your-username/claude-code-skills
    # Marketplace name comes from .claude-plugin/marketplace.json
 
    # Install and test
-   /plugin install productivity-skills@your-marketplace-name
+   claude plugin install productivity-skills@your-marketplace-name
    ```
 
 6. **Submit Pull Request:**