renovate: termix via customManagers (kubernetes manager doesn't honor inline comments)

The previous attempt (inline `# renovate:` comment in termix.yaml) silently
did nothing -- after merge + manual run, the dashboard's
`termix-system/termix.yaml (2)` was the resource count (Deployment +
Ingress), not detected updates. No PRs opened, no termix branches, no
queue entries anywhere.

Root cause: Renovate's `kubernetes` manager does NOT process inline
`# renovate:` comments. Those work for dockerfile/flux/helmfile/github-
actions/helm-values/etc., but kubernetes is missing from that list.

Correct fix: a `customManagers.regex` entry that extracts termix's image
directly with the right datasource/versioning/extractVersion set at
EXTRACTION time -- before any docker-version pre-check can reject the
prefixed tag. Plus a packageRule disabling the kubernetes manager for
termix so it doesn't silently skip the dep and clutter the dashboard.

Changes:
  - admin-system/renovate.yaml:
    * enabledManagers += "custom.regex"
    * customManagers: termix.yaml regex extraction -> github-releases
      datasource on Termix-SSH/Termix with `extractVersion=^release-(?<version>.+)$`
    * packageRules: disable kubernetes manager for ghcr.io/lukegus/termix
  - termix-system/termix.yaml: drop the useless inline comment, leave a
    NOTE explaining where the actual config lives.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

termix-system/termix.yaml

@@ -41,7 +41,10 @@ spec:
     spec:
       containers:
         - name: termix
-          # renovate: datasource=github-releases depName=Termix-SSH/Termix versioning=loose extractVersion=^release-(?<version>.+)$
+          # NOTE: termix uses a non-semver tag pattern (release-X.Y.Z).
+          # Renovate handles it via a customManagers regex defined in
+          # admin-system/renovate.yaml (the kubernetes manager doesn't
+          # process inline `# renovate:` comments).
           image: ghcr.io/lukegus/termix:release-1.11.0
           imagePullPolicy: IfNotPresent
           ports: