fix(observability): give loki an exec-form healthcheck in its image

The loki unit came up 'unhealthy' after the quadlet cutover. Cause: a unit's
HealthCmd reaches podman as CMD-SHELL, i.e. /bin/sh -c, and the loki image is
distroless — no shell exists in it. The busybox command itself works (probed
by hand: /ready answers 'ready'); it was the shell wrapper that could not run.
podman does not accept an exec-form command through --health-cmd either: it
stores a JSON array as one literal argument.

The command therefore moves into dankclaw/loki/Containerfile as a
HEALTHCHECK ... CMD [...] exec form, which is what that image exists for — it
already ships the static busybox solely so a healthcheck has something to run.
podman reads the image's healthcheck when the unit declares none.

scripts/test-changed.sh

@@ -209,6 +209,14 @@ for path in "${CHANGED[@]}"; do
             add_test tests/docs_completeness.bats
             add_test tests/deployment_scoping.bats
             ;;
+        # loki and alloy images are built from this repo's Containerfiles; the
+        # loki one carries the healthcheck command (see its comment), so both
+        # seams read it.
+        dankclaw/loki/*|dankclaw/alloy/*)
+            add_test tests/observability_quadlet.bats
+            add_test tests/caddyfile.bats
+            add_test tests/deployment_scoping.bats
+            ;;
         dankclaw/udev/*)
             # Host-level udev rules are applied by manual sudo install + udevadm
             # trigger, not dispatched as a service; nothing here is bats-testable.