Add better-model

[talkstream]

Summary

• Add better-model to 09. Meta & Orchestration
• Evidence-based model routing for Claude Code — ships custom agents (sonnet-coder, haiku-explorer) with model: frontmatter
• Routes 80% of tasks to Sonnet/Haiku instead of Opus. Zero dependencies, one command (npx better-model init)
• Field-tested: Sonnet usage increased from 1.6% to 21.1% across real Max subscriber sessions

Checklist

• Entry added to README.md in alphabetical order
• Format matches existing external tool entries (airis-mcp-gateway, pied-piper, taskade)
• Placed in appropriate category (09. Meta & Orchestration)