test: Add unit tests for formatTrackBeadID helper

Extract the cross-rig bead formatting logic into a testable helper
function and add comprehensive unit tests:

- TestFormatTrackBeadID: 8 test cases covering HQ beads, cross-rig
  beads, and edge cases (single segment, empty string, many segments)
- TestFormatTrackBeadIDConsumerCompatibility: 3 test cases verifying
  the external ref format can be correctly parsed by consumers in
  convoy.go, model.go, feed/convoy.go, and web/fetcher.go

The helper function includes godoc with examples showing expected
behavior for different bead ID formats.

internal/cmd/sling.go

@@ -1240,15 +1240,7 @@ func createAutoConvoy(beadID, beadTitle string) (string, error) {
 	}
 
 	// Add tracking relation: convoy tracks the issue
-	// 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)
-		}
-	}
+	trackBeadID := formatTrackBeadID(beadID)
 	depArgs := []string{"dep", "add", convoyID, trackBeadID, "--type=tracks"}
 	depCmd := exec.Command("bd", depArgs...)
 	depCmd.Dir = townBeads
@@ -1408,3 +1400,24 @@ func runBatchSling(beadIDs []string, rigName string, townBeadsDir string) error
 
 	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.
+//
+// Examples:
+//   - "hq-abc123" -> "hq-abc123" (HQ beads unchanged)
+//   - "gt-mol-xyz" -> "external:gt-mol:gt-mol-xyz"
+//   - "beads-task-123" -> "external:beads-task:beads-task-123"
+func formatTrackBeadID(beadID string) string {
+	if strings.HasPrefix(beadID, "hq-") {
+		return beadID
+	}
+	parts := strings.SplitN(beadID, "-", 3)
+	if len(parts) >= 2 {
+		rigPrefix := parts[0] + "-" + parts[1]
+		return fmt.Sprintf("external:%s:%s", rigPrefix, beadID)
+	}
+	// Fallback for malformed IDs (single segment)
+	return beadID
+}