hive-c0re: hive warnings reach the operator as metrics and alerts, not a banner

The dashboard's top-of-page warning banner is gone. The warning registry
stays, because /health/ready, the swarm status view and the OTEL gauges
read it. What it feeds is now alerted on in Grafana:

- hyperhive.hive.warning{kind,level}: one series per raised warning, so an
  alert names the cause. Raising a warning also logs one `hive warning
  raised` line with the message.
- hyperhive.hive.nix_store.utilization: the used fraction of the nix
  store's filesystem, probed in the gauge callback.
- Alerts: hh-hive-degraded on the existing degraded gauge, hh-hive-warning
  per kind (disk excluded), hh-host-disk-high at 85% and
  hh-host-disk-critical at 95%.
- pending_logins is now a registry warning, refreshed from the container
  rescan, so it reaches the per-kind gauge.

Frontend: server_warnings leaves the state snapshot, and the dashboard,
flow and stats pages keep only the sticky chrome wrapper. The shared
hive-warn and WarnBanner components stay, since other views use them.

A module-eval case checks each alert's metric name against the exporter's
source, so a rename on either side fails the build.

hive-c0re/src/lifecycle/mod.rs

@@ -367,11 +367,11 @@ pub async fn agents_for_meta_listing() -> Result<Vec<crate::meta::AgentSpec>> {
     agents_for_meta(None).await
 }
 
-/// Per-agent "stop failed" banner guards, keyed by agent name. Cleared the
+/// Per-agent "stop failed" warning guards, keyed by agent name. Cleared the
 /// next time that agent's container stops cleanly — a `Mutex<HashMap<...>>`
 /// rather than a bare `set_boot_warning` because this condition (unlike a
 /// one-shot boot step) genuinely resolves later, once an operator has
-/// intervened, and the banner should clear with it instead of surviving
+/// intervened, and the warning should clear with it instead of surviving
 /// until the next hive-c0re restart.
 fn stop_failed_guards()
 -> &'static std::sync::Mutex<std::collections::HashMap<String, crate::warnings::WarningGuard>> {
@@ -402,7 +402,7 @@ pub async fn kill(name: &str) -> Result<()> {
         .unwrap_or_else(std::sync::PoisonError::into_inner);
     match result {
         Ok(()) => {
-            // A clean stop clears any earlier "this looked wedged" banner
+            // A clean stop clears any earlier "this looked wedged" warning
             // for the same agent.
             guards.remove(name);
             Ok(())