docs: add skills.sh marketplace documentation (v3.11.5)

Added comprehensive documentation for skills.sh (Vercel Labs) marketplace:

Section 5.5 (Skills Marketplace):
- Installation instructions (npx add-skill <owner/repo>)
- Top skills by category (Frontend, Database, Auth, Testing)
- Format compatibility (100% compatible with .claude/skills/)
- Status & trade-offs (community project, early stage)

Section 11.3 (Skills Distribution Platforms):
- Comparison: skills.sh vs claude-code-templates vs SkillsMP
- When to use each platform (discovery, installation, team-specific)

Updated files:
- guide/ultimate-guide.md: New subsection Skills Marketplace (~line 5172)
- guide/ai-ecosystem.md: New section 11.3 Skills Distribution Platforms
- machine-readable/reference.yaml: Added 7 skills_marketplace_* entries
- README.md: Added skills.sh to Complementary Resources table
- CHANGELOG.md: Detailed entry for v3.11.5
- VERSION: Bumped to 3.11.5
- guide/cheatsheet.md: Version sync

Marketplace stats (Jan 23, 2026):
- 200+ skills available
- Top skill: vercel-react-best-practices (35K+ installs)
- 20+ agents supported (Claude Code, Cursor, Copilot, etc.)
- Launched: January 21, 2026 by Vercel Labs

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

VERSION

@@ -1 +1 @@
-3.11.4
+3.11.5