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>

CHANGELOG.md

@@ -4,6 +4,30 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
+## [3.11.5] - 2026-01-23
+
+### Added
+
+- **skills.sh marketplace documentation** (`guide/ultimate-guide.md:5172`, `guide/ai-ecosystem.md:1284`)
+  - New subsection in Section 5.5: "Skills Marketplace: skills.sh"
+  - Vercel Labs project (launched Jan 21, 2026): centralized skill discovery + one-command install
+  - 200+ skills, leaderboard with 35K+ installs (vercel-react-best-practices top skill)
+  - Format 100% compatible with `.claude/skills/` structure
+  - Installation: `npx add-skill <owner/repo>` → copies to `~/.claude/skills/`
+  - Supported agents: 20+ (Claude Code, Cursor, GitHub Copilot, Windsurf, etc.)
+  - Top skills by category: Frontend (vercel-react, web-design), Database (supabase-postgres), Auth (better-auth), Testing (TDD)
+  - Status documented: Community project (Vercel Labs, not official Anthropic), early stage
+  - Trade-offs: Centralized discovery vs GitHub distribution, multi-agent focus vs Claude Code specific
+  - Cross-reference added to `guide/ai-ecosystem.md` Section 11.3 (Skills Distribution Platforms)
+  - Complementary resources table updated in README.md
+  - YAML index entries:
+    - `skills_marketplace: 5172`
+    - `skills_marketplace_url: https://skills.sh/`
+    - `skills_marketplace_github: vercel-labs/agent-skills`
+    - `skills_marketplace_install: npx add-skill <owner/repo>`
+    - `skills_marketplace_top_skills` (5 entries with install counts)
+    - `skills_marketplace_status: Community (Vercel Labs), launched Jan 21, 2026`
+
 ## [3.11.4] - 2026-01-23
 
 ### Added