docs: overhaul homepage with tools bar, tabbed install, and professional polish
- Replace text-only platform list with styled color-coded tool badges
- Each badge links to the relevant install section
- Add tabbed Quick Install section (Claude Code / Codex / Gemini / Cursor)
- Restructure 'What's Inside' grid: skills, agents, personas, orchestration, tools, plugins, commands, multi-tool
- Rename 'POWERFUL' to 'Advanced' in domain cards
- Add tools-bar CSS with per-tool brand colors and hover effects
- Mobile-responsive tool badges
- Broaden site_name: 'Agent Skills' (not just Claude Code)
- Fix edit_uri: edit/main/docs/ (was missing docs/ prefix)
- Build verified: 238 sitemap URLs, all new pages included
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