Add collision resolution with automatic ID remapping

Implements --resolve-collisions flag for import command to safely handle ID
collisions during branch merges. When enabled, colliding issues are remapped
to new IDs and all text references and dependencies are automatically updated.

Also adds comprehensive tests, branch-merge example, and documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/main.go

@@ -229,6 +229,9 @@ var showCmd = &cobra.Command{
 		if issue.Design != "" {
 			fmt.Printf("\nDesign:\n%s\n", issue.Design)
 		}
+		if issue.Notes != "" {
+			fmt.Printf("\nNotes:\n%s\n", issue.Notes)
+		}
 		if issue.AcceptanceCriteria != "" {
 			fmt.Printf("\nAcceptance Criteria:\n%s\n", issue.AcceptanceCriteria)
 		}
@@ -350,6 +353,18 @@ var updateCmd = &cobra.Command{
 			assignee, _ := cmd.Flags().GetString("assignee")
 			updates["assignee"] = assignee
 		}
+		if cmd.Flags().Changed("design") {
+			design, _ := cmd.Flags().GetString("design")
+			updates["design"] = design
+		}
+		if cmd.Flags().Changed("notes") {
+			notes, _ := cmd.Flags().GetString("notes")
+			updates["notes"] = notes
+		}
+		if cmd.Flags().Changed("acceptance-criteria") {
+			acceptanceCriteria, _ := cmd.Flags().GetString("acceptance-criteria")
+			updates["acceptance_criteria"] = acceptanceCriteria
+		}
 
 		if len(updates) == 0 {
 			fmt.Println("No updates specified")
@@ -378,6 +393,9 @@ func init() {
 	updateCmd.Flags().IntP("priority", "p", 0, "New priority")
 	updateCmd.Flags().String("title", "", "New title")
 	updateCmd.Flags().StringP("assignee", "a", "", "New assignee")
+	updateCmd.Flags().String("design", "", "Design notes")
+	updateCmd.Flags().String("notes", "", "Additional notes")
+	updateCmd.Flags().String("acceptance-criteria", "", "Acceptance criteria")
 	rootCmd.AddCommand(updateCmd)
 }