refactor: separate Gitea and GitHub workflows

- Add .gitea/workflows/release.yml for Gitea builds
  - Uses plain docker commands (no action dependencies)
  - Pushes to git.prettyhefty.com registry
- Simplify .github/workflows/build.yaml for GitHub only
  - Remove Gitea detection logic
  - Only push latest tag for non-prerelease versions

.gitea/workflows/release.yml

@@ -0,0 +1,43 @@
+name: Build and Push Docker Image
+
+on:
+  push:
+    tags:
+      - 'v*.*.*'
+
+env:
+  REGISTRY: git.prettyhefty.com
+  IMAGE_NAME: bill/grist-mcp
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Extract version from tag
+        id: version
+        run: |
+          VERSION=${GITHUB_REF#refs/tags/}
+          echo "VERSION=$VERSION" >> $GITHUB_OUTPUT
+          if [[ "$VERSION" == *-alpha* ]] || [[ "$VERSION" == *-beta* ]] || [[ "$VERSION" == *-rc* ]]; then
+            echo "IS_PRERELEASE=true" >> $GITHUB_OUTPUT
+          else
+            echo "IS_PRERELEASE=false" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Log in to Container Registry
+        run: echo "${{ secrets.REGISTRY_TOKEN }}" | docker login ${{ env.REGISTRY }} -u ${{ gitea.actor }} --password-stdin
+
+      - name: Build and push Docker image
+        run: |
+          docker build -t ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ steps.version.outputs.VERSION }} .
+          docker push ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ steps.version.outputs.VERSION }}
+
+          if [ "${{ steps.version.outputs.IS_PRERELEASE }}" = "false" ]; then
+            docker tag ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ steps.version.outputs.VERSION }} ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest
+            docker push ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest
+          fi
+
+      - name: List images
+        run: docker images | grep grist-mcp