Add GitHub Actions workflow for tag-triggered DMG release

Builds Universal Binary (arm64 + x86_64), packages DMG with hdiutil,
and uploads as GitHub Release artifact when a v* tag is pushed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -0,0 +1,62 @@
+name: Release DMG
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+jobs:
+  build:
+    runs-on: macos-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Select Xcode
+        run: sudo xcode-select -s /Applications/Xcode.app/Contents/Developer
+
+      - name: Build Universal Binary
+        run: |
+          xcodebuild \
+            -project StockMonitor/StockMonitor.xcodeproj \
+            -scheme StockMonitor \
+            -configuration Release \
+            -derivedDataPath build \
+            ARCHS="arm64 x86_64" \
+            ONLY_ACTIVE_ARCH=NO \
+            CODE_SIGN_IDENTITY="-" \
+            CODE_SIGNING_REQUIRED=NO \
+            CODE_SIGNING_ALLOWED=NO
+
+      - name: Locate .app
+        id: locate
+        run: |
+          APP_PATH=$(find build -name "StockMonitor.app" -maxdepth 6 | head -1)
+          echo "app_path=$APP_PATH" >> "$GITHUB_OUTPUT"
+
+      - name: Create DMG
+        run: |
+          APP_PATH="${{ steps.locate.outputs.app_path }}"
+          VERSION="${GITHUB_REF_NAME}"
+          DMG_NAME="StockMonitor-${VERSION}.dmg"
+
+          mkdir -p dmg_staging
+          cp -R "$APP_PATH" dmg_staging/
+
+          hdiutil create \
+            -volname "StockMonitor ${VERSION}" \
+            -srcfolder dmg_staging \
+            -ov \
+            -format UDZO \
+            "$DMG_NAME"
+
+          echo "DMG created: $DMG_NAME"
+
+      - name: Upload Release
+        uses: softprops/action-gh-release@v2
+        with:
+          files: StockMonitor-*.dmg
+          generate_release_notes: true
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}