feat(rig): add route from rig beads to town beads

Add AppendRouteToDir helper and use it to add hq-* route during rig
initialization. This allows rig beads to resolve role beads and other
hq-* prefixed beads stored in town beads.

Uses safe append pattern (load, merge, write) instead of overwriting
to avoid clobbering future rig routes.

Supersedes PR #184 with proper implementation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/beads/routes.go

@@ -57,7 +57,12 @@ func LoadRoutes(beadsDir string) ([]Route, error) {
 // If the prefix already exists, it updates the path.
 func AppendRoute(townRoot string, route Route) error {
 	beadsDir := filepath.Join(townRoot, ".beads")
+	return AppendRouteToDir(beadsDir, route)
+}
 
+// AppendRouteToDir appends a route to routes.jsonl in the given beads directory.
+// If the prefix already exists, it updates the path.
+func AppendRouteToDir(beadsDir string, route Route) error {
 	// Load existing routes
 	routes, err := LoadRoutes(beadsDir)
 	if err != nil {