feat: update docs and blog — Amp SDK as native agent backend

[rubenmarcus]

Summary

Update documentation and write a blog post announcing Amp SDK as a native agent backend in ralph-starter.

Linear: ENG-1467

Documentation Updates

1. AGENTS.md: Add Amp with full setup instructions, mode descriptions
2. README.md: Update agent list, add Amp usage example
3. docs/: Add Amp page to Docusaurus site with:
   • Installation guide
   • Configuration options
   • Mode comparison (smart/rush/deep)
   • Performance comparison with other agents

Blog Post

• "ralph-starter now supports Amp by Sourcegraph"
• Agent comparison table (updated)
• Setup guide
• When to use Amp vs Claude Code vs other agents

Depends On

All other Amp issues (K-P) should be complete first.