feat(update): add --type flag to bd update command (GH#522)

Allow changing an issue type (bug, feature, task, epic, chore) via the
bd update command. The storage layer already supported issue_type in
allowedUpdateFields, this just exposes it through the CLI.

Changes:
- Add --type/-t flag to updateCmd in show.go
- Add IssueType field to UpdateArgs in protocol.go
- Handle issue_type in updatesFromArgs in server_issues_epics.go
- Add validation using ParseIssueType before update

Example usage:
  bd update ab-xyz --type epic

Fixes: #522

cmd/bd/show.go

@@ -526,6 +526,15 @@ var updateCmd = &cobra.Command{
 			setLabels, _ := cmd.Flags().GetStringSlice("set-labels")
 			updates["set_labels"] = setLabels
 		}
+		if cmd.Flags().Changed("type") {
+			issueType, _ := cmd.Flags().GetString("type")
+			// Validate issue type
+			if _, err := validation.ParseIssueType(issueType); err != nil {
+				fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+				os.Exit(1)
+			}
+			updates["issue_type"] = issueType
+		}
 
 		if len(updates) == 0 {
 			fmt.Println("No updates specified")
@@ -606,6 +615,9 @@ var updateCmd = &cobra.Command{
 				if setLabels, ok := updates["set_labels"].([]string); ok {
 					updateArgs.SetLabels = setLabels
 				}
+				if issueType, ok := updates["issue_type"].(string); ok {
+					updateArgs.IssueType = &issueType
+				}
 
 				resp, err := daemonClient.Update(updateArgs)
 				if err != nil {
@@ -1019,6 +1031,7 @@ func init() {
 	updateCmd.Flags().StringP("status", "s", "", "New status")
 	registerPriorityFlag(updateCmd, "")
 	updateCmd.Flags().String("title", "", "New title")
+	updateCmd.Flags().StringP("type", "t", "", "New type (bug|feature|task|epic|chore)")
 	registerCommonIssueFlags(updateCmd)
 	updateCmd.Flags().String("notes", "", "Additional notes")
 	updateCmd.Flags().String("acceptance-criteria", "", "DEPRECATED: use --acceptance")