chore: remove auto-merge stub (handled by nexus-merge-bot.sh)

The auto-merge.yml workflow was a noop stub that only printed
'See nexus-merge-bot.sh' on manual dispatch. Auto-merge is already
handled by the nexus-merge-bot.sh script running locally, making
this file dead code.

.gitea/workflows/auto-merge.yml

@@ -1,10 +0,0 @@
-# Placeholder — auto-merge is handled by nexus-merge-bot.sh
-# Gitea Actions requires a runner to be registered.
-# When a runner is available, this can replace the bot.
-name: stub
-on: workflow_dispatch
-jobs:
-  noop:
-    runs-on: ubuntu-latest
-    steps:
-      - run: echo "See nexus-merge-bot.sh"