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

cmd/active.go

@@ -0,0 +1,63 @@
+package cmd
+
+import (
+	"fmt"
+	"sort"
+
+	"github.com/spf13/cobra"
+	"github.com/warkanum/unitdore/config"
+	"github.com/warkanum/unitdore/systemd"
+)
+
+var activeCmd = &cobra.Command{
+	Use:   "active",
+	Short: "Enable and start all installed, enabled units",
+	Long: `Active runs 'systemctl enable --now' for all enabled units that have
+been installed. Units must be installed first via 'unitdore install'.`,
+	RunE: runActive,
+}
+
+func init() {
+	rootCmd.AddCommand(activeCmd)
+}
+
+func runActive(cmd *cobra.Command, args []string) error {
+	cfg, err := config.Load(configPath)
+	if err != nil {
+		return err
+	}
+
+	// Sort by order then name for deterministic startup sequence
+	units := make([]config.Unit, len(cfg.Units))
+	copy(units, cfg.Units)
+	sort.Slice(units, func(i, j int) bool {
+		if units[i].Order != units[j].Order {
+			return units[i].Order < units[j].Order
+		}
+		return units[i].Name < units[j].Name
+	})
+
+	started := 0
+	failed := 0
+
+	for _, u := range units {
+		if !u.Enabled {
+			continue
+		}
+		if !systemd.IsInstalled(u) {
+			fmt.Printf("  ! %s: not installed — run 'unitdore install' first\n", u.Name)
+			continue
+		}
+		fmt.Printf("  ▶ enabling: %s...\n", systemd.ServiceName(u))
+		if err := systemd.Enable(u); err != nil {
+			fmt.Printf("  ✗ failed: %s: %v\n", u.Name, err)
+			failed++
+		} else {
+			fmt.Printf("  ✓ started: %s\n", u.Name)
+			started++
+		}
+	}
+
+	fmt.Printf("\nDone. Started: %d  Failed: %d\n", started, failed)
+	return nil
+}