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

dankclaw/observability/quadlet/prometheus.container

@@ -27,10 +27,11 @@
 #
 # RULES. Five LLM alert rules and one recording rule load from
 # /etc/prometheus/rules/ (installed from shared/prometheus/rules/ by the
-# deployer). There is still **no Alertmanager**, so nothing is delivered: a
-# firing alert is visible in Prometheus and on the Grafana dashboard's ALERTS
-# panel and reaches nobody. Delivery is an operator decision, not an oversight;
-# the fragility is that an undelivered alert looks identical to a quiet one.
+# deployer). Delivery went live 2026-09-17: the alerting block in
+# prometheus.yml points at alertmanager:9093, which webhooks every firing
+# group to the host-side Signal relay (127.0.0.1:18093, reached as
+# host.containers.internal), which sends through signal-cli account 318311.
+# The Grafana dashboard's ALERTS panel remains the second surface.
 #
 # The config volume is also why rules belong in git: editing them in the UI is
 # lost on the next deploy, since the deployer overwrites the whole directory.