fix(sling): register hq-cv- prefix for convoy beads (#475)

Instead of changing the convoy ID format, register the hq-cv- prefix
as a valid route pointing to town beads. This preserves the semantic
meaning of convoy IDs (hq-cv-xxxxx) while fixing the prefix mismatch.

Changes:
- Register hq-cv- prefix during gt install
- Add doctor check and fix for missing convoy route
- Update routes_check tests for both hq- and hq-cv- routes

Fixes: gt-4nmfh

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/install.go

@@ -404,6 +404,12 @@ func initTownBeads(townPath string) error {
 		fmt.Printf("   %s Could not update routes.jsonl: %v\n", style.Dim.Render("⚠"), err)
 	}
 
+	// Register hq-cv- prefix for convoy beads (auto-created by gt sling).
+	// Convoys use hq-cv-* IDs for visual distinction from other town beads.
+	if err := beads.AppendRoute(townPath, beads.Route{Prefix: "hq-cv-", Path: "."}); err != nil {
+		fmt.Printf("   %s Could not register convoy prefix: %v\n", style.Dim.Render("⚠"), err)
+	}
+
 	return nil
 }