Implement merge validation logic (bd-192)

- Add merge command with --into and --dry-run flags
- Validate target and source issues exist
- Validate no self-merge attempts
- Add comprehensive test coverage
- Capture --id flag feature request as bd-9369

Amp-Thread-ID: https://ampcode.com/threads/T-22945597-9f4f-413b-afde-dcf3099eb2f0
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/merge.go

@@ -0,0 +1,126 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/fatih/color"
+	"github.com/spf13/cobra"
+)
+
+var mergeCmd = &cobra.Command{
+	Use:   "merge [source-id...] --into [target-id]",
+	Short: "Merge duplicate issues into a single issue",
+	Long: `Merge one or more source issues into a target issue.
+
+This command:
+1. Validates all issues exist and no self-merge
+2. Closes source issues with reason 'Merged into bd-X'
+3. Migrates all dependencies from sources to target
+4. Updates text references in all issue descriptions/notes
+
+Example:
+  bd merge bd-42 bd-43 --into bd-42
+  bd merge bd-10 bd-11 bd-12 --into bd-10 --dry-run`,
+	Args: cobra.MinimumNArgs(1),
+	Run: func(cmd *cobra.Command, args []string) {
+		targetID, _ := cmd.Flags().GetString("into")
+		if targetID == "" {
+			fmt.Fprintf(os.Stderr, "Error: --into flag is required\n")
+			os.Exit(1)
+		}
+
+		sourceIDs := args
+		dryRun, _ := cmd.Flags().GetBool("dry-run")
+
+		// Validate merge operation
+		if err := validateMerge(targetID, sourceIDs); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+
+		// TODO: Add RPC support when daemon implements MergeIssues
+		if daemonClient != nil {
+			fmt.Fprintf(os.Stderr, "Error: merge command not yet supported in daemon mode (see bd-190)\n")
+			os.Exit(1)
+		}
+
+		// Direct mode
+		ctx := context.Background()
+
+		if dryRun {
+			if !jsonOutput {
+				fmt.Println("Dry run - validation passed, no changes made")
+				fmt.Printf("Would merge: %s into %s\n", strings.Join(sourceIDs, ", "), targetID)
+			}
+			return
+		}
+
+		// Perform merge
+		if err := performMerge(ctx, targetID, sourceIDs); err != nil {
+			fmt.Fprintf(os.Stderr, "Error performing merge: %v\n", err)
+			os.Exit(1)
+		}
+
+		// Schedule auto-flush
+		markDirtyAndScheduleFlush()
+
+		if jsonOutput {
+			result := map[string]interface{}{
+				"target_id":  targetID,
+				"source_ids": sourceIDs,
+				"merged":     len(sourceIDs),
+			}
+			outputJSON(result)
+		} else {
+			green := color.New(color.FgGreen).SprintFunc()
+			fmt.Printf("%s Merged %d issue(s) into %s\n", green("✓"), len(sourceIDs), targetID)
+		}
+	},
+}
+
+func init() {
+	mergeCmd.Flags().String("into", "", "Target issue ID to merge into (required)")
+	mergeCmd.Flags().Bool("dry-run", false, "Validate without making changes")
+	rootCmd.AddCommand(mergeCmd)
+}
+
+// validateMerge checks that merge operation is valid
+func validateMerge(targetID string, sourceIDs []string) error {
+	ctx := context.Background()
+
+	// Check target exists
+	target, err := store.GetIssue(ctx, targetID)
+	if err != nil {
+		return fmt.Errorf("target issue not found: %s", targetID)
+	}
+	if target == nil {
+		return fmt.Errorf("target issue not found: %s", targetID)
+	}
+
+	// Check all sources exist and validate no self-merge
+	for _, sourceID := range sourceIDs {
+		if sourceID == targetID {
+			return fmt.Errorf("cannot merge issue into itself: %s", sourceID)
+		}
+
+		source, err := store.GetIssue(ctx, sourceID)
+		if err != nil {
+			return fmt.Errorf("source issue not found: %s", sourceID)
+		}
+		if source == nil {
+			return fmt.Errorf("source issue not found: %s", sourceID)
+		}
+	}
+
+	return nil
+}
+
+// performMerge executes the merge operation
+func performMerge(ctx context.Context, targetID string, sourceIDs []string) error {
+	// TODO: Implement actual merge logic in bd-190
+	// This is a placeholder for validation purposes
+	return fmt.Errorf("merge operation not yet implemented (see bd-190)")
+}