Phase 5-7: README, Makefile, unit tests

- README.md: full usage docs
- Makefile: build/install/uninstall/test/lint/clean targets
- config/config_test.go: 7 tests (load, save, find, add, roundtrip, invalid yaml)
- runtime/runtime_test.go: 6 tests (get, available, ListRunning graceful degradation)
- systemd/generator_test.go: 7 tests (ServiceName, Generate system/user/custom/docker/unknown, buildExecCommands)
- fix: docker/podman ListRunning returns nil (not error) when daemon unavailable
- fix: invalid YAML test uses tab-indent which is genuinely unparseable

Makefile

@@ -0,0 +1,45 @@
+BINARY     := unitdore
+INSTALL_DIR := /usr/local/bin
+CONFIG_DIR  := /etc/unitdore
+VERSION    := $(shell git describe --tags --always --dirty 2>/dev/null || echo "dev")
+LDFLAGS    := -ldflags "-X main.version=$(VERSION)"
+
+.PHONY: all build install uninstall test lint clean
+
+all: build
+
+## build: compile the binary
+build:
+	go build $(LDFLAGS) -o $(BINARY) .
+
+## install: install binary and create config dir
+install: build
+	install -Dm755 $(BINARY) $(INSTALL_DIR)/$(BINARY)
+	mkdir -p $(CONFIG_DIR)
+	@echo "Installed to $(INSTALL_DIR)/$(BINARY)"
+	@echo "Config dir: $(CONFIG_DIR)"
+
+## uninstall: remove binary
+uninstall:
+	rm -f $(INSTALL_DIR)/$(BINARY)
+	@echo "Removed $(INSTALL_DIR)/$(BINARY)"
+
+## test: run all unit tests
+test:
+	go test ./... -v
+
+## test-short: run tests without verbose output
+test-short:
+	go test ./...
+
+## lint: run go vet
+lint:
+	go vet ./...
+
+## clean: remove built binary
+clean:
+	rm -f $(BINARY)
+
+## help: show this help
+help:
+	@grep -E '^## ' Makefile | sed 's/## /  /'