Add make check target and CI workflow (#36)

Adds a `make check` target that verifies formatting (gofmt), linting (golangci-lint), and tests (go test -race) without modifying files.

Also adds `.gitea/workflows/check.yml` CI workflow that runs on pushes and PRs to main and next.

`make check` passes cleanly on current next branch.

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #36
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

.gitea/workflows/check.yml

@@ -0,0 +1,13 @@
+name: check
+on:
+  push:
+    branches: [main, next]
+  pull_request:
+    branches: [main, next]
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13894f8d5 # v4
+      - name: Build Docker image (runs make check internally)
+        run: docker build .