Improve DEV-LOCAL.md with prerequisites, verification steps, and troubleshooting

[renezander030]

Summary

Improves the local development guide (DEV-LOCAL.md) to make it easier for new contributors and self-hosters to get a working local environment.

• Prerequisites section: OS, CPU, RAM, Go version, Docker, gsutil, KVM check
• Verification steps after each major step (expected output, how to confirm it worked)
• Required vs optional services: documents that the observability stack (Grafana, Loki, Tempo, Mimir, etc.) is optional for basic operation
• Hello world section: curl and Python SDK examples to verify end-to-end sandbox creation
• Environment files: documents all .env.local files and their purpose
• Troubleshooting section: KVM not found, port conflicts, sudo requirements, gsutil install, ClickHouse timing, huge pages

Closes #2211 (partial — covers the local dev documentation portion).

[claude]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Point client-proxy health check to the health listener

The verification step uses http://localhost:3002/health, but run-local starts client-proxy with default ports where ProxyPort is 3002 and HealthPort is 3003 (packages/client-proxy/internal/cfg/model.go), and main.go binds the health server to HealthPort. In a normal local setup this check can fail even when the service is healthy, which makes the guide report a false negative for a working client-proxy instance.

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