feat: Add label operations to bd update command

Implements bd-au0.2, completing all P0 tasks in the command standardization epic.

Changes:
- Add --add-label, --remove-label, --set-labels flags to bd update
- Support multiple labels via repeatable flags
- Implement in both daemon and direct modes
- Add comprehensive tests for all label operations

The bd update command now supports:
  bd update <id> --add-label <label>       # Add one or more labels
  bd update <id> --remove-label <label>    # Remove one or more labels
  bd update <id> --set-labels <labels>     # Replace all labels

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

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

cmd/bd/show.go

@@ -393,6 +393,18 @@ var updateCmd = &cobra.Command{
 			externalRef, _ := cmd.Flags().GetString("external-ref")
 			updates["external_ref"] = externalRef
 		}
+		if cmd.Flags().Changed("add-label") {
+			addLabels, _ := cmd.Flags().GetStringSlice("add-label")
+			updates["add_labels"] = addLabels
+		}
+		if cmd.Flags().Changed("remove-label") {
+			removeLabels, _ := cmd.Flags().GetStringSlice("remove-label")
+			updates["remove_labels"] = removeLabels
+		}
+		if cmd.Flags().Changed("set-labels") {
+			setLabels, _ := cmd.Flags().GetStringSlice("set-labels")
+			updates["set_labels"] = setLabels
+		}
 
 		if len(updates) == 0 {
 			fmt.Println("No updates specified")
@@ -461,6 +473,15 @@ var updateCmd = &cobra.Command{
 				if externalRef, ok := updates["external_ref"].(string); ok { // NEW: Map external_ref
 					updateArgs.ExternalRef = &externalRef
 				}
+				if addLabels, ok := updates["add_labels"].([]string); ok {
+					updateArgs.AddLabels = addLabels
+				}
+				if removeLabels, ok := updates["remove_labels"].([]string); ok {
+					updateArgs.RemoveLabels = removeLabels
+				}
+				if setLabels, ok := updates["set_labels"].([]string); ok {
+					updateArgs.SetLabels = setLabels
+				}
 
 				resp, err := daemonClient.Update(updateArgs)
 				if err != nil {
@@ -488,9 +509,63 @@ var updateCmd = &cobra.Command{
 		// Direct mode
 		updatedIssues := []*types.Issue{}
 		for _, id := range resolvedIDs {
-			if err := store.UpdateIssue(ctx, id, updates, actor); err != nil {
-				fmt.Fprintf(os.Stderr, "Error updating %s: %v\n", id, err)
-				continue
+			// Apply regular field updates if any
+			regularUpdates := make(map[string]interface{})
+			for k, v := range updates {
+				if k != "add_labels" && k != "remove_labels" && k != "set_labels" {
+					regularUpdates[k] = v
+				}
+			}
+			if len(regularUpdates) > 0 {
+				if err := store.UpdateIssue(ctx, id, regularUpdates, actor); err != nil {
+					fmt.Fprintf(os.Stderr, "Error updating %s: %v\n", id, err)
+					continue
+				}
+			}
+
+			// Handle label operations
+			// Set labels (replaces all existing labels)
+			if setLabels, ok := updates["set_labels"].([]string); ok && len(setLabels) > 0 {
+				// Get current labels
+				currentLabels, err := store.GetLabels(ctx, id)
+				if err != nil {
+					fmt.Fprintf(os.Stderr, "Error getting labels for %s: %v\n", id, err)
+					continue
+				}
+				// Remove all current labels
+				for _, label := range currentLabels {
+					if err := store.RemoveLabel(ctx, id, label, actor); err != nil {
+						fmt.Fprintf(os.Stderr, "Error removing label %s from %s: %v\n", label, id, err)
+						continue
+					}
+				}
+				// Add new labels
+				for _, label := range setLabels {
+					if err := store.AddLabel(ctx, id, label, actor); err != nil {
+						fmt.Fprintf(os.Stderr, "Error setting label %s on %s: %v\n", label, id, err)
+						continue
+					}
+				}
+			}
+
+			// Add labels
+			if addLabels, ok := updates["add_labels"].([]string); ok {
+				for _, label := range addLabels {
+					if err := store.AddLabel(ctx, id, label, actor); err != nil {
+						fmt.Fprintf(os.Stderr, "Error adding label %s to %s: %v\n", label, id, err)
+						continue
+					}
+				}
+			}
+
+			// Remove labels
+			if removeLabels, ok := updates["remove_labels"].([]string); ok {
+				for _, label := range removeLabels {
+					if err := store.RemoveLabel(ctx, id, label, actor); err != nil {
+						fmt.Fprintf(os.Stderr, "Error removing label %s from %s: %v\n", label, id, err)
+						continue
+					}
+				}
 			}
 
 			if jsonOutput {
@@ -822,6 +897,9 @@ func init() {
 	updateCmd.Flags().String("notes", "", "Additional notes")
 	updateCmd.Flags().String("acceptance-criteria", "", "DEPRECATED: use --acceptance")
 	_ = updateCmd.Flags().MarkHidden("acceptance-criteria")
+	updateCmd.Flags().StringSlice("add-label", nil, "Add labels (repeatable)")
+	updateCmd.Flags().StringSlice("remove-label", nil, "Remove labels (repeatable)")
+	updateCmd.Flags().StringSlice("set-labels", nil, "Set labels, replacing all existing (repeatable)")
 
 	updateCmd.Flags().Bool("json", false, "Output JSON format")
 	rootCmd.AddCommand(updateCmd)