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/issue-opened.yml

@@ -1,21 +0,0 @@
-name: 'Label Issues'
-
-on:
-  issues:
-    types: ['opened', 'reopened']
-
-jobs:
-  label_issues:
-    runs-on: hanzo-build-linux-amd64
-    permissions:
-      issues: write
-    steps:
-      - uses: actions/github-script@v6
-        with:
-          script: |
-            github.rest.issues.addLabels({
-              issue_number: context.issue.number,
-              owner: context.repo.owner,
-              repo: context.repo.repo,
-              labels: ["status: triage"]
-            })