add swarm-metrics, an agent CLI for the metrics store

swarm-metrics reads the swarm's VictoriaMetrics from inside an agent
container: query, query-range, series, labels and label-values, each one
GET against the store's Prometheus read API through the gateway, printing
the store's JSON. It authenticates the way swarm-logs does, with the
agent's own queue secret read from bao under the agent's leaf, sent as
a swarm-agent bearer.

On the metrics store's host the read API's auth map gains an agent arm
ahead of the Grafana one. It routes to swarm-metrics-agent-auth, a
swarm-logs-auth instance logged in under the existing swarm-metrics-auth
leaf, whose bao grant now includes swarm/agents/+/queue read. The check is
rate-limited per address like the log store's
(deploy.victoriametrics.agentAuth{Rate,Burst,VerdictTtl}).

nix/agent-modules/metrics.nix installs the wrapped binary when the hive
forwards HIVE_AGENT_METRICS_URL (from swarm.victoriametrics.domain) and a
store address, the same gate as swarm-logs. Adds the base:swarm-metrics
skill and the CLI reference with its freshness check.

Fixes #5395

nix/host-modules/hive-c0re/environment.nix

@@ -287,6 +287,9 @@ in
   # `swarm-victorialogs.nix`'s location for the whole reasoning.
   lib.optionalAttrs haveBaoClientIdentity {
     HIVE_AGENT_LOGS_QUERY_URL = "https://${config.services.hyperhive.swarm.victorialogs.domain}/select/logsql/query";
+    # The metrics store's origin, for `swarm-metrics`; same credential and
+    # gate. Its read locations answer a bare 401, never a login redirect.
+    HIVE_AGENT_METRICS_URL = "https://${config.services.hyperhive.swarm.victoriametrics.domain}";
   }
 //
   # Where the swarm's secret store is, and the identity this hive presents to