feat(infra): migrate container registry from SCW to internal GitLab

Full migration off Scaleway Container Registry to internal GitLab
registry backed by MinIO S3. All 4 images (ci-builder, ci-builder-cpu,
foxhunt-runtime, foxhunt-training-runtime) rebuilt in internal registry.

Registry & images:
- All image refs → gitlab-registry.foxhunt.svc.cluster.local:5000/root/foxhunt/
- imagePullSecrets: scw-registry → gitlab-registry
- Kaniko build template: two-step DAG (git-clone → kaniko-build) with shared PVC
- Kaniko layer cache enabled at root/foxhunt/cache
- AWS_ACCESS_KEY_ID: $SCW_ACCESS_KEY → $MINIO_ACCESS_KEY in .gitlab-ci.yml

Network policies:
- ci-pipeline: add HTTP/80, registry/5000, webservice/8181 egress rules

DNS & Tailscale proxy cleanup:
- Remove ci, prometheus, monitor DNS records (no longer exposed)
- Rename s3 → minio DNS record
- Remove Argo UI, Prometheus, monitor nginx server blocks
- Remove argo-htpasswd volume mount
- Tailscale proxy nodeSelector: infra → platform

Terraform cleanup:
- Delete infra/modules/registry/ (SCW CR namespace)
- Delete infra/modules/object-storage/ (SCW S3 buckets)
- Delete infra/modules/secrets/ (SCW secrets)
- Delete corresponding live configs
- TF state backend: S3 → GitLab HTTP

Argo workflows:
- Add events/ (GitLab push eventsource + ci-pipeline sensor)
- ci-pipeline + training templates: SCW → internal registry
- Delete obsolete compile-training-template.yaml

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.devcontainer/devcontainer.json

@@ -4,7 +4,7 @@
     "dockerfile": "Dockerfile",
     "context": "..",
     "args": {
-      "CI_BUILDER_IMAGE": "rg.fr-par.scw.cloud/foxhunt-ci/ci-builder:latest"
+      "CI_BUILDER_IMAGE": "gitlab-registry.foxhunt.svc.cluster.local:5000/root/foxhunt/ci-builder:latest"
     }
   },
   "remoteUser": "dev",