Phase 3+4: systemd generator, install, active, status

- systemd/generator.go: build .service file content from Unit
- systemd/manager.go: install/uninstall/enable/disable/status via systemctl
- cmd/install.go: write unit files, --dry-run flag, remove disabled units
- cmd/active.go: enable + start units in order
- cmd/status.go: summary table with name/runtime/user/enabled/installed/state

systemd/generator.go

@@ -0,0 +1,105 @@
+package systemd
+
+import (
+	"fmt"
+	"strings"
+	"text/template"
+
+	"github.com/warkanum/unitdore/config"
+)
+
+const systemUnitTemplate = `# /etc/systemd/system/{{.ServiceName}}
+# Generated by unitdore — do not edit manually
+
+[Unit]
+Description=Unitdore: {{.Unit.Name}} ({{.Unit.Runtime}})
+After=network.target
+
+[Service]
+Type=simple
+ExecStart={{.ExecStart}}
+ExecStop={{.ExecStop}}
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target
+`
+
+const userUnitTemplate = `# ~/.config/systemd/user/{{.ServiceName}}
+# Generated by unitdore — do not edit manually
+
+[Unit]
+Description=Unitdore: {{.Unit.Name}} ({{.Unit.Runtime}})
+After=default.target
+
+[Service]
+Type=simple
+ExecStart={{.ExecStart}}
+ExecStop={{.ExecStop}}
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=default.target
+`
+
+type templateData struct {
+	ServiceName string
+	Unit        config.Unit
+	ExecStart   string
+	ExecStop    string
+}
+
+// ServiceName returns the systemd service name for a unit.
+func ServiceName(u config.Unit) string {
+	return fmt.Sprintf("unitdore-%s.service", u.Name)
+}
+
+// Generate produces the .service file content for a unit.
+func Generate(u config.Unit) (string, error) {
+	execStart, execStop := buildExecCommands(u)
+
+	data := templateData{
+		ServiceName: ServiceName(u),
+		Unit:        u,
+		ExecStart:   execStart,
+		ExecStop:    execStop,
+	}
+
+	tmplStr := systemUnitTemplate
+	if u.User != "" {
+		tmplStr = userUnitTemplate
+	}
+
+	tmpl, err := template.New("unit").Parse(tmplStr)
+	if err != nil {
+		return "", fmt.Errorf("parsing template: %w", err)
+	}
+
+	var buf strings.Builder
+	if err := tmpl.Execute(&buf, data); err != nil {
+		return "", fmt.Errorf("rendering template: %w", err)
+	}
+	return buf.String(), nil
+}
+
+func buildExecCommands(u config.Unit) (start, stop string) {
+	// If a custom command is provided, use it directly
+	if u.Command != "" {
+		return u.Command, ""
+	}
+
+	switch u.Runtime {
+	case "podman":
+		start = fmt.Sprintf("/usr/bin/podman start -a %s", u.Name)
+		stop = fmt.Sprintf("/usr/bin/podman stop %s", u.Name)
+	case "docker":
+		start = fmt.Sprintf("/usr/bin/docker start %s", u.Name)
+		stop = fmt.Sprintf("/usr/bin/docker stop %s", u.Name)
+	default:
+		start = fmt.Sprintf("/usr/bin/%s start %s", u.Runtime, u.Name)
+		stop = fmt.Sprintf("/usr/bin/%s stop %s", u.Runtime, u.Name)
+	}
+	return start, stop
+}