feat(service): cmd.service takes a status action

[skeeeon]

cmd.service gains a status action, so you can read one service's state on demand. Before, the only way to see it was to wait for the next service_check telemetry.

nats request "agents.server-01.cmd.service" '{"action":"status","service_name":"nginx"}'

{"status":"success","service_name":"nginx","action":"status",
 "result":"Service nginx is Running","service_status":"Running","ts":"..."}

Design

• One lookup. QueryService calls GetServiceStatuses, the function the service_check telemetry already uses on every platform. The command and the telemetry can't describe the same service differently, and there's no new per-platform code.
• One gate. The service must be in allowed_services, the same as start, stop and restart.
• Same words as telemetry. service_status uses the telemetry's values: Running, Stopped, Starting, Stopping, Error, Unknown or NotInstalled.
• Missing services aren't errors. A service that doesn't exist answers NotInstalled, as it does in telemetry.
• Control replies are unchanged. service_status is absent for start, stop and restart, and the reply is otherwise the same shape as before.

Testing

• Every platform: an unlisted service is refused before anything asks the service manager.
• Linux, against the real systemd: systemd-journald reports Running, and a made-up service reports NotInstalled. The test skips on machines without systemd.
• Full suite:
  • go test -race ./... passes
  • go vet is clean on Linux, Windows, FreeBSD and darwin
  • all four release targets build
  • golangci-lint shows nothing new
• Not covered: the handler's routing branch (about ten lines). Its reply can't be captured without a live NATS connection, so only the executor side is tested.

Known limit

On Windows, status connects to the service manager the same way the telemetry does. That works when the agent runs as a service. From a non-admin shell it answers "Access is denied", just as the telemetry would.

Also

Fixes the example reply in docs/architecture.md's service diagram. It showed an output field that cmd.service has never sent; the field is result.

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