ci(agent): publish the GPU agent image (build-agent job)

Build + push fabledcurator-agent alongside web/ml (own CUDA + onnxruntime-gpu
image, context=agent/, same tag cadence: main → :main/:latest/:c-<sha>, tag →
:<version>). So the operator PULLS + runs it on the GPU machine instead of
building locally. README switched to docker pull.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

.forgejo/workflows/build.yml

@@ -329,3 +329,41 @@ jobs:
           file: Dockerfile.ml
           push: true
           tags: ${{ steps.tag.outputs.tags }}
+
+  # The desktop GPU agent (#114) — published so the operator pulls + runs it on
+  # the GPU machine instead of building locally. Independent of web/ml (its own
+  # CUDA + onnxruntime-gpu image, context = agent/). Same tag cadence.
+  build-agent:
+    runs-on: python-ci
+    container:
+      image: git.fabledsword.com/bvandeusen/ci-python:3.14
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Determine tag
+        id: tag
+        run: |
+          SHORT_SHA=$(printf '%s' "$GITHUB_SHA" | cut -c1-7)
+          if [ "${GITHUB_REF#refs/tags/}" != "${GITHUB_REF}" ]; then
+            TAG_NAME="${GITHUB_REF#refs/tags/}"
+            echo "tags=git.fabledsword.com/bvandeusen/fabledcurator-agent:${TAG_NAME}" >> "$GITHUB_OUTPUT"
+          elif [ "${GITHUB_REF##*/}" = "main" ]; then
+            echo "tags=git.fabledsword.com/bvandeusen/fabledcurator-agent:main,git.fabledsword.com/bvandeusen/fabledcurator-agent:latest,git.fabledsword.com/bvandeusen/fabledcurator-agent:c-${SHORT_SHA}" >> "$GITHUB_OUTPUT"
+          else
+            echo "tags=git.fabledsword.com/bvandeusen/fabledcurator-agent:dev" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Login to Forgejo registry
+        uses: docker/login-action@v3
+        with:
+          registry: git.fabledsword.com
+          username: ${{ github.actor }}
+          password: ${{ secrets.RELEASE_TOKEN }}
+
+      - name: Build and push agent image
+        uses: docker/build-push-action@v5
+        with:
+          context: agent
+          file: agent/Dockerfile
+          push: true
+          tags: ${{ steps.tag.outputs.tags }}