fix(sling): Format cross-rig beads as external refs in convoy tracking

When creating auto-convoys for cross-rig beads (e.g., gt-xxx or gu-xxx),
the tracking relation was failing because bd couldn't resolve the bead ID
from HQ context. Now formats non-HQ beads as external:prefix:id for proper
resolution.

Fixes convoy tracking for cross-rig sling operations.

internal/cmd/sling.go

@@ -1240,7 +1240,16 @@ func createAutoConvoy(beadID, beadTitle string) (string, error) {
 	}
 
 	// Add tracking relation: convoy tracks the issue
-	depArgs := []string{"dep", "add", convoyID, beadID, "--type=tracks"}
+	// Format cross-prefix beads as external refs so bd can resolve them
+	trackBeadID := beadID
+	if !strings.HasPrefix(beadID, "hq-") {
+		parts := strings.SplitN(beadID, "-", 3)
+		if len(parts) >= 2 {
+			rigPrefix := parts[0] + "-" + parts[1]
+			trackBeadID = fmt.Sprintf("external:%s:%s", rigPrefix, beadID)
+		}
+	}
+	depArgs := []string{"dep", "add", convoyID, trackBeadID, "--type=tracks"}
 	depCmd := exec.Command("bd", depArgs...)
 	depCmd.Dir = townBeads
 	depCmd.Stderr = os.Stderr