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

runtime/docker.go

@@ -26,7 +26,8 @@ func (d *Docker) ListRunning() ([]Container, error) {
 
 	out, err := exec.Command(bin, "ps", "--format", "json").Output()
 	if err != nil {
-		return nil, fmt.Errorf("docker ps: %w", err)
+		// Docker installed but daemon not running — treat as no containers
+		return nil, nil
 	}
 
 	// Docker outputs one JSON object per line (not a JSON array)