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

scripts/deploy.sh

@@ -138,6 +138,7 @@ valid_service() {
         dankclaw:gitea-server|dankclaw:homepage|\
         dankclaw:comfyui|dankclaw:hermes|dankclaw:hermes-dashboard|\
         dankclaw:hermes-webui|dankclaw:prometheus|dankclaw:grafana|dankclaw:loki|dankclaw:alloy|\
+        dankclaw:alertmanager|\
         dankclaw:node-exporter|dankclaw:authentik-postgres|dankclaw:authentik-server|\
         dankclaw:authentik-worker|dankclaw:authentik-outpost|dankclaw:authentik-outpost-ldap|dankclaw:anubis|dankclaw:seerr|\
         dankclaw:frank-large|dankclaw:frank-small|\