alerting: Alertmanager + Signal relay deliver firing rules to Frank's phone

- dankclaw/observability/quadlet/alertmanager.container: loopback :9093,
  config from shared/prometheus/alertmanager.yml, family conventions
- shared/prometheus/alertmanager.yml: single signal receiver, group_by
  alertname, 30s/5m/4h intervals, webhook to host.containers.internal:18093
- scripts/signal-relay.py + .service: host-side user unit, loopback :18093,
  15-min dedupe, sends via signal-cli account 318311
- prometheus.yml: alerting block to alertmanager:9093; stale comments fixed
- llm-fleet.yml: delivery header; LlmSpecDecodeOff annotation now drafter-agnostic
- deploy.sh/deploy-remote.sh/deploy-plan.py: alertmanager service, config
  copy, relay install, planner routing for config + relay files
- homepage: alertmanager status card (loopback probe)
- tests: alertmanager in UNITS/loopback/health/PAIRS/planner + five new
  delivery-path seams (webhook<->relay contract, prometheus target, relay
  unit, signal-cli account, planner routing)
- docs: Alertmanager.md component page, Observability.md, Prometheus.md,
  index.md, AGENTS.md delivery line

shared/homepage/services.json

@@ -186,6 +186,17 @@
     "wip": false,
     "showLink": false
   },
+  {
+    "name": "alertmanager",
+    "category": "admin",
+    "description": "Alert delivery to Signal.",
+    "url": "",
+    "docs": "",
+    "probe": "127.0.0.1:9093",
+    "method": "http",
+    "wip": false,
+    "showLink": false
+  },
   {
     "name": "Caddy",
     "category": "admin",