test(observability): copier sandboxes, probe-target seam exception, cop gates

- caddyfile seam learns the documented exception: URL-shaped targets
  are legal only inside a prober job (state is per-job; the first cut
  leaked it and passed a planted URL).
- observability seams: blackbox unit + planner entries, both copiers
  executed for real in sandboxes, alloy engine-validation refusal
  (image-gated), dashboard panel invariants.
- AGENTS.md: structural target downs retired.

AGENTS.md

@@ -176,11 +176,12 @@ alertmanager.container`, loopback :9093) → host-side Signal relay
 (`scripts/signal-relay.py`, user unit, 192.168.0.143:18093 — the host LAN
 IP, the one bind a container can reach as `host.containers.internal`;
 iptables admits loopback, the host and the podman bridge subnet only) → signal-cli account 318311.
-Prometheus targets read down for structural reasons (HTML-scraped "probes",
-Gitea metrics off); the remedy is a blackbox exporter. `frank-zealot` was down for
-a different reason until 2026-10-18: llama.cpp will not serve `/metrics` at all
-without `--metrics`. See
-`serverAppConfigs-docs/Infrastructure/Observability.md`.
+The former structural target downs (HTML-scraped "probes", Gitea metrics off)
+are retired as of 2026-10-21: HTTP liveness rides a blackbox exporter
+(`shared/blackbox/blackbox.yml`), Gitea serves `/metrics` from its quadlet, and
+every Prometheus job reports up. `frank-zealot` was down for a different reason
+until 2026-10-18: llama.cpp will not serve `/metrics` at all without
+`--metrics`. See `serverAppConfigs-docs/Infrastructure/Observability.md`.
 
 ## Testing