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/packages/default.nix

@@ -43,6 +43,8 @@ let
     # which is exactly what this list's per-bin packages are for — see
     # `nix/agent-modules/logs.nix` for what puts it on PATH there.
     swarm-logs = "hyperhive agent-facing swarm log store query CLI";
+    # The same for the metrics store; `nix/agent-modules/metrics.nix` puts it on PATH.
+    swarm-metrics = "hyperhive agent-facing swarm metrics store query CLI";
     hive-screen-mcp = "hyperhive screen MCP bridge (screenshot + input for GUI agents)";
     hive-forge = "hyperhive Forgejo CLI";
     hive-forge-notify = "hyperhive per-agent Forgejo notification poller daemon";