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/sling_convoy.go

@@ -65,7 +65,8 @@ func createAutoConvoy(beadID, beadTitle string) (string, error) {
 
 	townBeads := filepath.Join(townRoot, ".beads")
 
-	// Generate convoy ID with cv- prefix
+	// Generate convoy ID with hq-cv- prefix for visual distinction
+	// The hq-cv- prefix is registered in routes during gt install
 	convoyID := fmt.Sprintf("hq-cv-%s", slingGenerateShortID())
 
 	// Create convoy with title "Work: <issue-title>"