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

@@ -114,9 +114,9 @@ func init() {
 	slingCmd.Flags().StringVar(&slingAccount, "account", "", "Claude Code account handle to use")
 	slingCmd.Flags().StringVar(&slingAgent, "agent", "", "Override agent/runtime for this sling (e.g., claude, gemini, codex, or custom alias)")
 	slingCmd.Flags().BoolVar(&slingNoConvoy, "no-convoy", false, "Skip auto-convoy creation for single-issue sling")
-	slingCmd.Flags().BoolVar(&slingHookRawBead, "hook-raw-bead", false, "Hook raw bead without default formula (expert mode)")
 	slingCmd.Flags().StringVar(&slingEpic, "epic", "", "Link auto-created convoy to parent epic")
 	slingCmd.Flags().StringVar(&slingConvoy, "convoy", "", "Add to existing convoy instead of creating new")
+	slingCmd.Flags().BoolVar(&slingHookRawBead, "hook-raw-bead", false, "Hook raw bead without default formula (expert mode)")
 
 	rootCmd.AddCommand(slingCmd)
 }
@@ -380,9 +380,21 @@ 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 {

internal/cmd/sling_convoy.go

@@ -109,6 +109,45 @@ func createAutoConvoy(beadID, beadTitle string) (string, error) {
 	return convoyID, nil
 }
 
+// addToExistingConvoy adds a bead to an existing convoy by creating a tracking relation.
+// Returns an error if the convoy doesn't exist or the tracking relation fails.
+func addToExistingConvoy(convoyID, beadID string) error {
+	townRoot, err := workspace.FindFromCwd()
+	if err != nil {
+		return fmt.Errorf("finding town root: %w", err)
+	}
+
+	townBeads := filepath.Join(townRoot, ".beads")
+	dbPath := filepath.Join(townBeads, "beads.db")
+
+	// Verify convoy exists and is open
+	query := fmt.Sprintf(`
+		SELECT id FROM issues
+		WHERE id = '%s'
+		AND issue_type = 'convoy'
+		AND status = 'open'
+	`, convoyID)
+
+	queryCmd := exec.Command("sqlite3", dbPath, query)
+	out, err := queryCmd.Output()
+	if err != nil || strings.TrimSpace(string(out)) == "" {
+		return fmt.Errorf("convoy %s not found or not open", convoyID)
+	}
+
+	// Add tracking relation: convoy tracks the issue
+	trackBeadID := formatTrackBeadID(beadID)
+	depArgs := []string{"--no-daemon", "dep", "add", convoyID, trackBeadID, "--type=tracks"}
+	depCmd := exec.Command("bd", depArgs...)
+	depCmd.Dir = townBeads
+	depCmd.Stderr = os.Stderr
+
+	if err := depCmd.Run(); err != nil {
+		return fmt.Errorf("adding tracking relation: %w", err)
+	}
+
+	return nil
+}
+
 // formatTrackBeadID formats a bead ID for use in convoy tracking dependencies.
 // Cross-rig beads (non-hq- prefixed) are formatted as external references
 // so the bd tool can resolve them when running from HQ context.