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.

tests/caddyfile.bats

@@ -193,7 +193,18 @@ CADDYFILE="$REPO_ROOT/dankclaw/Caddyfile"
 }
 
 @test "prometheus targets are host:port (no URL schemes)" {
-    ! grep -nE '^\s*-\s*"?http' "$REPO_ROOT/shared/prometheus/prometheus.yml"
+    # Plain scrape targets are host:port; a URL target is only meaningful for
+    # the blackbox prober job, whose relabel_configs copy it to __param_target
+    # and point __address__ at the prober. Anywhere else a URL target is the
+    # old HTML-probe defect wearing a new hat.
+    awk '
+        /job_name:/ { job=$0; sub(/.*"/, "", job); sub(/".*/, "", job); probe = 0 }
+        /metrics_path: \/probe/ { probe = 1 }
+        /^[[:space:]]*-[[:space:]]*"http/ && !probe {
+            print FILENAME ":" FNR ": URL target outside a prober job: " $0; bad = 1
+        }
+        END { exit bad }
+    ' "$REPO_ROOT/shared/prometheus/prometheus.yml"
 }
 
 @test "observability containers run as container root (rootless = host user)" {