ci: remove GitHub CI — native git.hanzo.ai + cd.hanzo.ai only

This workflow ran on a self-hosted actions-runner-controller pool. ARC is
decommissioned: CI executes inside git.hanzo.ai (Gitea Actions, .hanzo/workflows)
and delivery is cd.hanzo.ai reconciling the reviewed image pin in
hanzoai/universe. GitHub is a mirror and runs nothing.

Removed rather than repointed — there is no GitHub-side pool to move to, and
leaving it would keep ARC load-bearing.

.github/workflows/labeller.yml

@@ -1,12 +0,0 @@
-name: "Pull Request Labeler"
-on:
-  pull_request_target:
-    types: [opened, reopened]
-
-jobs:
-  triage:
-    runs-on: hanzo-build-linux-amd64
-    steps:
-    - uses: actions/labeler@v3
-      with:
-        repo-token: "${{ secrets.GITHUB_TOKEN }}"