feat(ci): Add Gitea Actions workflow with Nix caching

Uses johno/gitea-actions/nix-setup composite action for:
- Nix installation via DeterminateSystems/nix-installer-action
- Nix store caching via actions/cache@v4
- Per-repo cache isolation based on flake.lock hash

.gitea/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: johno/gitea-actions/nix-setup@main
+
+      - name: Check flake
+        run: nix flake check