ci(canonize): prune the Gitea remote, or deleted branches never disappear

Observed rather than reasoned: after the leftover rotation branch was deleted on
both remotes, pipeline 492 still reported it. The runner recycles its workspace
under GIT_STRATEGY=fetch, and git fetch without --prune keeps remote-tracking refs
for branches that no longer exist upstream. The job would have kept naming a branch
nobody can find, indefinitely - and before the previous commit it would have kept
failing on it, with deletion offering no way out.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitlab-ci.yml

@@ -72,7 +72,12 @@ canonize_rotation:
     - git config --global user.email "ci@axion1337.chat"
     - git config --global user.name "TURN-Rotation (automatische Kanonisierung)"
     - git remote add gitea https://rohana.axion1337.de/sorb/axion1337.chat-gitops.git
-    - git fetch --quiet gitea
+    # --prune ist Pflicht, nicht Kosmetik: der Runner recycelt seinen Workspace
+    # (GIT_STRATEGY=fetch), und ohne prune bleiben remote-tracking-Refs geloeschter
+    # Zweige stehen. Der Job saehe einen laengst entfernten Rotationszweig sonst
+    # dauerhaft weiter - live beobachtet am 2026-08-18, Pipeline 492: beide Remotes
+    # waren nachweislich leer, der Job meldete den Zweig trotzdem.
+    - git fetch --quiet --prune gitea
     # Kein 'fetch origin' noetig: der Runner hat main schon ausgecheckt (GIT_DEPTH 0),
     # CI_COMMIT_SHA ist der Stand von main beim Anlegen der Pipeline.
     - git checkout -B main "$CI_COMMIT_SHA"