Add README to examples/gymcoach

[skokaina]

Problem

examples/gymcoach/ contains a production-grade agent.yaml with 10 tools, memory, guardrails, evaluation, and observability — but no README. GitHub browsers (majority of HN/Reddit traffic) click examples/ first and have no way to know what this is or how to use it.

Proposed fix

Add examples/gymcoach/README.md covering:

1. What this agent does (1 sentence)
2. Required env vars (GROQ_API_KEY, DATABASE_URL, REDIS_URL, etc.)
3. Quick start: agentspec generate agent.yaml --framework langgraph --output ./agent/
4. Required services (Redis, Postgres, Langfuse)
5. Link to the migration guide at docs/guides/migrate-gymcoach.md

Acceptance criteria

• README exists with copy-pasteable setup instructions
• A new user can go from clone to generated code in under 2 minutes