feat: add stack up/down commands for navigation

cmd/down.go

@@ -0,0 +1,88 @@
+package cmd
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+
+	"github.com/javoire/stackinator/internal/git"
+	"github.com/javoire/stackinator/internal/stack"
+	"github.com/spf13/cobra"
+)
+
+var downCmd = &cobra.Command{
+	Use:   "down",
+	Short: "Move to a child branch in the stack",
+	Long: `Checkout a child branch of the current branch in the stack.
+
+If the current branch has no children (is at the tip of the stack),
+an error message will be displayed.
+
+If there are multiple children, you will be prompted to select one.`,
+	Example: `  # Move to child branch
+  stack down`,
+	Run: func(cmd *cobra.Command, args []string) {
+		gitClient := git.NewGitClient()
+
+		if err := runDown(gitClient); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+	},
+}
+
+func runDown(gitClient git.GitClient) error {
+	// Get current branch
+	currentBranch, err := gitClient.GetCurrentBranch()
+	if err != nil {
+		return fmt.Errorf("failed to get current branch: %w", err)
+	}
+
+	// Get children of current branch
+	children, err := stack.GetChildrenOf(gitClient, currentBranch)
+	if err != nil {
+		return fmt.Errorf("failed to get children: %w", err)
+	}
+
+	if len(children) == 0 {
+		return fmt.Errorf("no children (tip of stack)")
+	}
+
+	var targetBranch string
+
+	if len(children) == 1 {
+		// Only one child, checkout directly
+		targetBranch = children[0].Name
+	} else {
+		// Multiple children, prompt for selection
+		fmt.Printf("Multiple children found for %s:\n", currentBranch)
+		for i, child := range children {
+			fmt.Printf("  %d) %s\n", i+1, child.Name)
+		}
+		fmt.Print("\nSelect branch (1-" + strconv.Itoa(len(children)) + "): ")
+
+		reader := bufio.NewReader(os.Stdin)
+		input, err := reader.ReadString('\n')
+		if err != nil {
+			return fmt.Errorf("failed to read input: %w", err)
+		}
+
+		input = strings.TrimSpace(input)
+		selection, err := strconv.Atoi(input)
+		if err != nil || selection < 1 || selection > len(children) {
+			return fmt.Errorf("invalid selection: %s", input)
+		}
+
+		targetBranch = children[selection-1].Name
+	}
+
+	// Checkout the target branch
+	if err := gitClient.CheckoutBranch(targetBranch); err != nil {
+		return fmt.Errorf("failed to checkout child branch %s: %w", targetBranch, err)
+	}
+
+	fmt.Printf("Switched to child branch: %s\n", targetBranch)
+	return nil
+}

cmd/root.go

@@ -72,6 +72,8 @@ func init() {
 	rootCmd.AddCommand(renameCmd)
 	rootCmd.AddCommand(reparentCmd)
 	rootCmd.AddCommand(worktreeCmd)
+	rootCmd.AddCommand(upCmd)
+	rootCmd.AddCommand(downCmd)
 }
 
 // Execute runs the root command

cmd/up.go

@@ -0,0 +1,51 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/javoire/stackinator/internal/git"
+	"github.com/spf13/cobra"
+)
+
+var upCmd = &cobra.Command{
+	Use:   "up",
+	Short: "Move to the parent branch in the stack",
+	Long: `Checkout the parent branch of the current branch in the stack.
+
+If the current branch has no parent (is at the root of the stack),
+an error message will be displayed.`,
+	Example: `  # Move to parent branch
+  stack up`,
+	Run: func(cmd *cobra.Command, args []string) {
+		gitClient := git.NewGitClient()
+
+		if err := runUp(gitClient); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+	},
+}
+
+func runUp(gitClient git.GitClient) error {
+	// Get current branch
+	currentBranch, err := gitClient.GetCurrentBranch()
+	if err != nil {
+		return fmt.Errorf("failed to get current branch: %w", err)
+	}
+
+	// Get parent from git config
+	parent := gitClient.GetConfig(fmt.Sprintf("branch.%s.stackparent", currentBranch))
+
+	if parent == "" {
+		return fmt.Errorf("already at stack root (no parent for %s)", currentBranch)
+	}
+
+	// Checkout the parent branch
+	if err := gitClient.CheckoutBranch(parent); err != nil {
+		return fmt.Errorf("failed to checkout parent branch %s: %w", parent, err)
+	}
+
+	fmt.Printf("Switched to parent branch: %s\n", parent)
+	return nil
+}