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_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.