claude support for agent

[jbchen]

Summary

• add Codex-to-Claude runtime fallback in the worker when Codex cannot be spawned
• surface worker runtime selection and fallback events through the router and CLI INFO logs
• document Claude fallback environment variables for providers

Testing

• python3 -m py_compile agent/codex_worker.py router/router.py
• direct worker smoke test with CODESWARM_CODEX_BIN=missing_codex and CODESWARM_CLAUDE_BIN=$PWD/claude