feat: cross-platform documentation and distribution

[nathanschram]

Phase 6: Documentation & Distribution (v1.11.0)

Context

With platform packages built, users need clear installation instructions per tool, and packages need distribution channels.

Deliverables

1. docs/guides/cross-platform.md — Per-platform installation and setup guide
2. Updated README.md — Add cross-platform installation section
3. Updated AGENTS.md — Note cross-platform support
4. npm package — Publish for Gemini CLI extension installation (npm install contextdocs)
5. CI validation — GitHub Actions job to verify platform packages stay in sync with core skills

Documentation Structure

• Getting started per platform (Claude Code, Gemini CLI, Copilot, Cursor, Codex CLI, OpenCode, Cline, Windsurf, Aider)
• Feature availability matrix per platform
• Migration guide for users switching between tools
• FAQ: "Which platform should I use?"

Verification

• All platform installation instructions tested
• npm package installs and works on Gemini CLI
• CI job catches sync drift between core and platform packages
• README, AGENTS.md, and llms.txt consistent

Part of the Cross-Platform ContextDocs Strategy. Depends on #19, #20, #21.