Add Gitea Actions CI workflow for pull request builds

.gitea/workflows/ci.yaml

@@ -0,0 +1,27 @@
+name: CI
+
+on:
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: ubuntu:24.04
+    steps:
+      - name: Install build dependencies
+        run: |
+          apt-get update
+          apt-get install -y --no-install-recommends \
+            build-essential \
+            libsdl2-dev \
+            libsdl2-image-dev \
+            libsdl2-mixer-dev \
+            pkg-config
+
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Build (native Linux)
+        run: make