feat: set default BUILDERNET_IMAGE to url and add readyz.sh helper script

[fkondej]

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[claude]

Claude finished @fkondej's task in 1m 3s —— View job

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Two issues flagged as inline comments:

1. readyz.sh missing QEMU liveness check — Unlike the readiness loop in start.sh, the new readyz.sh doesn't verify the QEMU process is still alive. If the VM crashes, it will silently poll for the full timeout (300s) instead of failing fast.

2. Hardcoded dev image URL in playground.yaml — The default BUILDERNET_IMAGE points to a specific dev build (dev-20260303-3059b761). This will go stale and need frequent bumps. A latest redirect URL would be more maintainable.

Everything else (README updates, script structure, env variable handling) looks good.