feat(sling): implement --convoy flag logic (gt-9o4)

Add --convoy flag to gt sling that allows adding an issue to an existing
convoy instead of creating a new one. When specified:
- Validates the convoy exists and is open
- Adds tracking relation between convoy and issue
- Skips auto-convoy creation

Changes:
- Add slingConvoy variable and --convoy flag registration
- Add addToExistingConvoy() helper function in sling_convoy.go
- Modify auto-convoy logic to check slingConvoy first

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

internal/cmd/sling.go

@@ -377,9 +377,21 @@ func runSling(cmd *cobra.Command, args []string) error {
 		}
 	}
 
-	// Auto-convoy: check if issue is already tracked by a convoy
-	// If not, create one for dashboard visibility (unless --no-convoy is set)
-	if !slingNoConvoy && formulaName == "" {
+	// Convoy handling: --convoy adds to existing, otherwise auto-create (unless --no-convoy)
+	if slingConvoy != "" {
+		// Use existing convoy specified by --convoy flag
+		if slingDryRun {
+			fmt.Printf("Would add to convoy %s\n", slingConvoy)
+			fmt.Printf("Would add tracking relation to %s\n", beadID)
+		} else {
+			if err := addToExistingConvoy(slingConvoy, beadID); err != nil {
+				return fmt.Errorf("adding to convoy: %w", err)
+			}
+			fmt.Printf("%s Added to convoy %s\n", style.Bold.Render("→"), slingConvoy)
+		}
+	} else if !slingNoConvoy && formulaName == "" {
+		// Auto-convoy: check if issue is already tracked by a convoy
+		// If not, create one for dashboard visibility
 		existingConvoy := isTrackedByConvoy(beadID)
 		if existingConvoy == "" {
 			if slingDryRun {