Add mina-healthcheck CLI for daemon health probes. Lightweight tool that queries the daemon's GraphQL API to check sync status, peer count, chain length, and combined readiness. Supports Kubernetes exec probes, Docker HEALTHCHECK, and a blocking wait command for init containers and CI pipelines. Includes mina_healthcheck_lib for programmatic use from OCaml code. The JSON error envelope contains only an `error` field (no `healthy` field) — consumers should distinguish success vs. failure by inspecting the JSON shape (presence of `error`) or the process exit code.
