build: containerize the control layer and publish it to GHCR

Add a second image for the handler control CLI (spawn/list/kill, approve/
reject, forge-init, CI poller) alongside the existing API image. It shares
the package, database, and /var/lib/handler volume but runs the control
process instead of uvicorn.

- Dockerfile.control: git + tmux baked in for live spawning; default CMD is
  the `poll-ci --watch` loop; RUN_MIGRATIONS toggle reuses docker-entrypoint.sh.
- docker-control.yml: builds/pushes ghcr.io/<repo>/control (multi-arch),
  scoped gha cache so it doesn't clobber the API build.
- docker-compose.yml: new `control` service, RUN_MIGRATIONS=false, depends on
  the API (which owns migrations) being healthy.
- README: Containers section documenting both images and compose usage.

.github/workflows/docker-control.yml

@@ -0,0 +1,68 @@
+name: docker-control
+
+# Builds the control-layer image (the `handler` CLI / CI poller) from Dockerfile.control
+# and publishes it to GHCR alongside the API image built by docker.yml.
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*"]
+  pull_request:
+    branches: [main]
+
+env:
+  IMAGE: ghcr.io/${{ github.repository }}/control
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to GHCR
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      # Tags: branch name on branch pushes, semver on v* tags, short SHA always,
+      # `latest` only on the default branch. metadata-action lowercases the repo
+      # name (GHCR requires lowercase).
+      - name: Docker metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.IMAGE }}
+          tags: |
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=sha
+            type=raw,value=latest,enable={{is_default_branch}}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: Dockerfile.control
+          platforms: linux/amd64,linux/arm64
+          # PRs build (to catch Dockerfile breakage) but never push.
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha,scope=control
+          cache-to: type=gha,mode=max,scope=control