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

.github/workflows/build.yaml

@@ -6,7 +6,8 @@ on:
       - 'v*.*.*'
 
 env:
-  IMAGE_NAME: grist-mcp
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
 
 jobs:
   build:
@@ -19,53 +20,27 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v3
-
-      - name: Determine registry
-        id: registry
-        run: |
-          if [ "${GITEA_ACTIONS}" = "true" ]; then
-            # Gitea: use server URL as registry
-            REGISTRY="${{ github.server_url }}"
-            REGISTRY="${REGISTRY#https://}"
-            REGISTRY="${REGISTRY#http://}"
-            echo "registry=${REGISTRY}" >> $GITHUB_OUTPUT
-            echo "is_gitea=true" >> $GITHUB_OUTPUT
-          else
-            # GitHub: use GHCR
-            echo "registry=ghcr.io" >> $GITHUB_OUTPUT
-            echo "is_gitea=false" >> $GITHUB_OUTPUT
-          fi
-
-      - name: Log in to GitHub Container Registry
-        if: steps.registry.outputs.is_gitea == 'false'
+      - name: Log in to Container Registry
         uses: docker/login-action@v3
         with:
-          registry: ghcr.io
+          registry: ${{ env.REGISTRY }}
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Log in to Gitea Container Registry
-        if: steps.registry.outputs.is_gitea == 'true'
-        uses: docker/login-action@v3
-        with:
-          registry: ${{ steps.registry.outputs.registry }}
-          username: ${{ github.actor }}
-          password: ${{ secrets.REGISTRY_TOKEN }}
-
-      - name: Extract metadata (tags, labels)
+      - name: Extract metadata for Docker
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ${{ steps.registry.outputs.registry }}/${{ github.repository }}
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
           tags: |
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
-            type=semver,pattern={{major}}
-            type=raw,value=latest
+            type=raw,value=latest,enable=${{ !contains(github.ref, '-alpha') && !contains(github.ref, '-beta') && !contains(github.ref, '-rc') }}
 
-      - name: Build and push
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build and push Docker image
         uses: docker/build-push-action@v6
         with:
           context: .