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
being 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/pr-labeler.yml

@@ -1,20 +0,0 @@
-name: 'PR Labeler'
-
-on:
-  - pull_request_target
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
-  cancel-in-progress: true
-
-jobs:
-  labeler:
-    permissions:
-      contents: read
-      pull-requests: write
-    runs-on: hanzo-build-linux-amd64
-    steps:
-      - uses: actions/labeler@v4
-        with:
-          repo-token: ${{ secrets.GITHUB_TOKEN }}
-          sync-labels: ''