fix: split Dockerfile with pinned images and add CI workflow (#14)

## Summary

Rewrites the Dockerfile to use sha256-pinned images and proper multi-stage build structure. Adds missing Makefile targets and a Gitea CI workflow.

## Changes

### Dockerfile
- **Lint stage**: `golangci/golangci-lint` v1.64.8 pinned by sha256 — runs `make fmt-check` + `make lint`
- **Test stage**: `golang` 1.22.12 pinned by sha256 — runs `make test` with dependency on lint stage
- Removed redundant final stage (this is a library with no binary to build)
- Both images pinned by digest with version+date comments

### Makefile
- Added `fmt-check` target: verifies `gofmt` compliance without modifying files
- Added `check` target: runs `fmt-check`, `lint`, `test` in sequence
- Added `hooks` target: installs a pre-commit hook that runs `make check`
- Separated `gofmt` check from `lint` target (was previously bundled)
- Changed default target from `test` to `check`

### CI
- Added `.gitea/workflows/check.yml`: runs `docker build .` on push to main and on PRs

## Verification

`docker build --progress plain .` passes — all stages complete successfully.

closes #9

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Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #14
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

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."