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/install.go

@@ -0,0 +1,83 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+	"github.com/warkanum/unitdore/config"
+	"github.com/warkanum/unitdore/systemd"
+)
+
+var dryRun bool
+
+var installCmd = &cobra.Command{
+	Use:   "install",
+	Short: "Generate and install systemd unit files for all enabled units",
+	Long: `Install generates .service files for all enabled units and writes them
+to the appropriate systemd directory. It does not enable or start them.
+
+Use --dry-run to preview the generated unit files without writing anything.`,
+	RunE: runInstall,
+}
+
+func init() {
+	installCmd.Flags().BoolVar(&dryRun, "dry-run", false, "preview unit files without writing")
+	rootCmd.AddCommand(installCmd)
+}
+
+func runInstall(cmd *cobra.Command, args []string) error {
+	cfg, err := config.Load(configPath)
+	if err != nil {
+		return err
+	}
+
+	installed := 0
+	skipped := 0
+	removed := 0
+
+	for _, u := range cfg.Units {
+		if !u.Enabled {
+			// Remove unit file if it exists for disabled units
+			if systemd.IsInstalled(u) {
+				if dryRun {
+					fmt.Printf("  ~ would remove: %s\n", systemd.ServiceName(u))
+				} else {
+					if err := systemd.Uninstall(u); err != nil {
+						fmt.Printf("  ✗ failed to remove %s: %v\n", u.Name, err)
+					} else {
+						fmt.Printf("  - removed: %s (disabled)\n", systemd.ServiceName(u))
+						removed++
+					}
+				}
+			} else {
+				skipped++
+			}
+			continue
+		}
+
+		if dryRun {
+			content, err := systemd.Generate(u)
+			if err != nil {
+				fmt.Printf("  ✗ %s: %v\n", u.Name, err)
+				continue
+			}
+			path, _ := systemd.UnitPath(u)
+			fmt.Printf("\n--- %s ---\n%s\n", path, content)
+			installed++
+			continue
+		}
+
+		if err := systemd.Install(u); err != nil {
+			fmt.Printf("  ✗ failed: %s: %v\n", u.Name, err)
+		} else {
+			path, _ := systemd.UnitPath(u)
+			fmt.Printf("  ✓ installed: %s\n", path)
+			installed++
+		}
+	}
+
+	if !dryRun {
+		fmt.Printf("\nDone. Installed: %d  Removed: %d  Skipped: %d\n", installed, removed, skipped)
+	}
+	return nil
+}