Add Makefile with build, test, lint, run, and clean targets

Standard targets:
- build: compiles cmd/librenotes with version/buildtime ldflags
- test: race detector enabled, full module
- lint: go vet, plus staticcheck if available
- run: build + execute, ARGS forwarded
- clean: remove binary and test/coverage artifacts

Variables (BINARY, OUTDIR, GO, GOFLAGS, LDFLAGS, TESTFLAGS) are
overridable so the CI workflow (#5) can invoke targets with
custom output paths or flags.

Closes #38.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Makefile

@@ -0,0 +1,37 @@
+BINARY      ?= librenotes
+OUTDIR      ?= .
+CMD_PKG     ?= ./cmd/librenotes
+GO          ?= go
+GOFLAGS     ?=
+LDFLAGS     ?= -X git.librete.ch/public/librenotes/internal/notesium.gitversion=$(shell git describe --tags --always --dirty 2>/dev/null || echo dev) \
+               -X git.librete.ch/public/librenotes/internal/notesium.buildtime=$(shell date -u +%Y-%m-%dT%H:%M:%SZ)
+TESTFLAGS   ?= -race -count=1
+
+.PHONY: all build test lint run clean help
+
+all: build
+
+build:
+	$(GO) build $(GOFLAGS) -ldflags "$(LDFLAGS)" -o $(OUTDIR)/$(BINARY) $(CMD_PKG)
+
+test:
+	$(GO) test $(TESTFLAGS) ./...
+
+lint:
+	$(GO) vet ./...
+	@command -v staticcheck >/dev/null 2>&1 && staticcheck ./... || echo "staticcheck not installed, skipping"
+
+run: build
+	$(OUTDIR)/$(BINARY) $(ARGS)
+
+clean:
+	rm -f $(OUTDIR)/$(BINARY)
+	rm -f *.test *.out coverage.out
+
+help:
+	@echo "Targets:"
+	@echo "  build  - compile binary to \$$OUTDIR/\$$BINARY (default ./librenotes)"
+	@echo "  test   - run all Go tests with race detector"
+	@echo "  lint   - run go vet (+ staticcheck if installed)"
+	@echo "  run    - build and run; pass args via ARGS=\"...\""
+	@echo "  clean  - remove build artifacts"