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/build.yml

@@ -1,50 +0,0 @@
-name: "Build docker image"
-
-on:
-  pull_request:
-    branches: ["main"]
-    types: [opened, synchronize, reopened, ready_for_review]
-
-concurrency:
-  group: build-${{ github.ref }}
-  cancel-in-progress: true
-
-jobs:
-  build_docker:
-    name: Build Docker Image
-    runs-on: hanzo-build-linux-amd64
-    if: github.event.pull_request.draft == false
-    steps:
-      - name: ⬇️ Checkout
-        uses: actions/checkout@v4
-        with:
-          fetch-depth: 2
-
-      - name: ⛅ Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-
-      - name: 🎁 Cache Docker layers
-        uses: actions/cache@v4
-        with:
-          path: /tmp/.buildx-cache
-          key: ${{ runner.os }}-buildx-${{ github.sha }}
-          restore-keys: |
-            ${{ runner.os }}-buildx-
-
-      - name: 🚀 Build Docker Image
-        uses: docker/build-push-action@v5
-        with:
-          push: false
-          context: .
-          file: ./docker/Dockerfile
-          tags: captable-${{ github.sha }}
-          cache-from: type=local,src=/tmp/.buildx-cache
-          cache-to: type=local,dest=/tmp/.buildx-cache-new,mode=max
-
-      - # Temp fix
-        # https://github.com/docker/build-push-action/issues/252
-        # https://github.com/moby/buildkit/issues/1896
-        name: 🚚 Move cache
-        run: |
-          rm -rf /tmp/.buildx-cache
-          mv /tmp/.buildx-cache-new /tmp/.buildx-cache