feat(convoy): default owner to creator when not specified

When --owner flag is not provided on gt convoy create, the owner now
defaults to the creator's identity (via detectSender()) rather than
being left empty. This ensures completion notifications always go to
the right place - the agent who requested the convoy.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/convoy.go

@@ -299,8 +299,14 @@ func runConvoyCreate(cmd *cobra.Command, args []string) error {
 
 	// Create convoy issue in town beads
 	description := fmt.Sprintf("Convoy tracking %d issues", len(trackedIssues))
-	if convoyOwner != "" {
-		description += fmt.Sprintf("\nOwner: %s", convoyOwner)
+
+	// Default owner to creator identity if not specified
+	owner := convoyOwner
+	if owner == "" {
+		owner = detectSender()
+	}
+	if owner != "" {
+		description += fmt.Sprintf("\nOwner: %s", owner)
 	}
 	if convoyNotify != "" {
 		description += fmt.Sprintf("\nNotify: %s", convoyNotify)
@@ -365,8 +371,8 @@ func runConvoyCreate(cmd *cobra.Command, args []string) error {
 	if len(trackedIssues) > 0 {
 		fmt.Printf("  Issues:   %s\n", strings.Join(trackedIssues, ", "))
 	}
-	if convoyOwner != "" {
-		fmt.Printf("  Owner:    %s\n", convoyOwner)
+	if owner != "" {
+		fmt.Printf("  Owner:    %s\n", owner)
 	}
 	if convoyNotify != "" {
 		fmt.Printf("  Notify:   %s\n", convoyNotify)