Initial commit: unitdore scaffold + syncup/edit commands

- config: load/save/add unit, Unit struct
- runtime: podman + docker discovery and exists check
- cmd: syncup (discover + reconcile), edit, cobra root
- PLAN.md: full project plan

cmd/edit.go

@@ -0,0 +1,34 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+
+	"github.com/spf13/cobra"
+)
+
+var editCmd = &cobra.Command{
+	Use:   "edit",
+	Short: "Open the units config in $EDITOR",
+	RunE:  runEdit,
+}
+
+func init() {
+	rootCmd.AddCommand(editCmd)
+}
+
+func runEdit(cmd *cobra.Command, args []string) error {
+	editor := os.Getenv("EDITOR")
+	if editor == "" {
+		editor = "vi"
+	}
+	c := exec.Command(editor, configPath)
+	c.Stdin = os.Stdin
+	c.Stdout = os.Stdout
+	c.Stderr = os.Stderr
+	if err := c.Run(); err != nil {
+		return fmt.Errorf("editor exited with error: %w", err)
+	}
+	return nil
+}