CI: move workflow to .github and use a subpath-aware checkout

actions/checkout drops the /git subpath of our Gitea when building the
clone URL. Use ischanx/checkout v6.0.2, pinned to its commit.

.github/workflows/ci.yml

@@ -0,0 +1,32 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*"]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node: [22, 24]
+    steps:
+      # Upstream actions/checkout builds the clone URL from the server's origin
+      # and drops a subpath such as /git (actions/checkout#1370, #1242), so it
+      # cannot clone from imsc.uni-graz.at/git. ischanx/checkout keeps the path.
+      # Pinned to the commit of its v6.0.2 tag; its v4 tags lack the fix.
+      - uses: ischanx/checkout@b3a7a36c84c480d7e1a6f9f7ca7bb55dc519f0ba # v6.0.2
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 11
+      - uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node }}
+          cache: pnpm
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm run ci