Add agent docs, website, skill, and concise README

[static-var]

Summary

• add agent-focused usage/API docs at docs/agent-usage.md
• add static GitHub Pages site under site/ with CNAME for ap.staticvar.dev
• add Pages deployment workflow
• add repo skill for AgentPreview-assisted Compose UI iteration
• rewrite README to be short and focused
• update sample READMEs to prefer real capture, with fake renderer as debug mode
• document sanitized snapshot output paths and platform-prefixed viewport folders

Website deployment notes

• GitHub Pages source should be set to GitHub Actions
• DNS for ap.staticvar.dev should CNAME to static-var.github.io
• site/CNAME contains ap.staticvar.dev

Verification

• Markdown sanity/link checks were run by subagents
• static site file/anchor/workflow checks were run by subagents
• final review subagent approved with no findings
• git diff --check 8d52cf8..HEAD

Review workflow

• Four implementation subagents worked independently on docs, website, skill, and README cleanup
• Review subagent found output path and .pi tracking issues
• Fixed docs to use sanitized preview ids and platform-prefixed viewport folders
• Added .gitignore unignore rules for .pi/agents/skills/**
• Final reviews approved