fix: split Dockerfile with pinned images and add CI workflow

- Pin golangci-lint to v1.64.8 by sha256 digest
- Pin golang to 1.22.12 by sha256 digest
- Lint stage runs make fmt-check + make lint
- Test stage runs make test with dependency on lint stage
- Remove redundant final stage (library has no binary)
- Add fmt-check, check, hooks targets to Makefile
- Add .gitea/workflows/check.yml for CI

closes #9

Makefile

@@ -1,6 +1,6 @@
-.PHONY: test
+.PHONY: test fmt fmt-check lint check docker hooks
 
-default: test
+default: check
 
 test:
 	@go test -v ./...
@@ -9,9 +9,20 @@ fmt:
 	goimports -l -w .
 	golangci-lint run --fix
 
+fmt-check:
+	@test -z "$$(gofmt -l .)" || { echo "gofmt would reformat:"; gofmt -l .; exit 1; }
+
 lint:
 	golangci-lint run
-	sh -c 'test -z "$$(gofmt -l .)"'
+
+check: fmt-check lint test
 
 docker:
 	docker build --progress plain .
+
+hooks:
+	@echo "Installing git hooks..."
+	@mkdir -p .git/hooks
+	@printf '#!/bin/sh\nmake check\n' > .git/hooks/pre-commit
+	@chmod +x .git/hooks/pre-commit
+	@echo "Pre-commit hook installed."