feat(bootstrap): host every built image in the internal registry — GC-durable pulls

The disk-pressure drill's dead end: kubelet's image GC collects an unused image
and an air-gapped node has nothing to pull it from (ImagePullBackOff until an
operator re-imports). The registry the bundle already renders becomes that pull
source:

- Every image the installer builds is now a registry ref
  (registry.felis.svc:5000/felis/{felis,limbo,lobby,paper}:demo), imported into
  containerd under that exact name (first boot needs no registry round-trip)
  and mirrored into the registry after deploy_bundle (push_image_to_registry:
  push endpoint 127.0.0.1:5000, and only the path after the host matters to the
  registry — a push there lands where kubelet's mirrored pull looks). A ref
  outside the registry is warned about, not silently unmirrored.

- configure_registry_mirror writes /etc/rancher/k3s/registries.yaml mapping
  registry.felis.svc:5000 onto http://127.0.0.1:5000, the loopback hostPort the
  registry Deployment binds (node containerd cannot dial the Service VIP — live
  drill: "Empty reply"). k3s regenerates containerd config only at agent start,
  so a CONTENT change restarts k3s and an identical file (every re-run)
  restarts nothing.

- import_registry_image caches registry:2 into containerd so the registry
  Deployment can start on a box that cannot reach Docker Hub.

- Migration 0021 re-points the recommended whitelist seeds ('felis-lobby:demo',
  'felis-paper:demo') at the registry refs — a user server created from those
  rows must not strand when GC collects the bare tag. Only recommended rows
  still holding the old seed are touched; enabled is preserved; a pre-existing
  target row wins over a duplicate.

bootstrap_test.sh pins the mirror idempotence (identical content must NOT
restart k3s), the push-ref mapping (including the port-confusion refusal) and
the registry:2 precheck.

internal/config/config.go

@@ -126,8 +126,9 @@ type RegistryConfig struct {
 	// install there IS no such reach (the build egress policy allows only DNS,
 	// the internal registry and explicit package mirrors), so the operator must
 	// point these at whatever their box can actually pull — typically images
-	// imported into the node's containerd alongside the felis image. Empty keeps
-	// the default.
+	// mirrored into the in-cluster registry (docs/troubleshooting.md §8e); a
+	// bare node-containerd import does not survive an image GC, there is no pull
+	// source for it. Empty keeps the default.
 	KanikoImage   string `toml:"kaniko_image"`
 	TrivyImage    string `toml:"trivy_image"`
 	BuildCPULimit string `toml:"build_cpu_limit"`