fix(beads): ensure directory exists before writing routes.jsonl

WriteRoutes() would fail if the beads directory didn't exist yet.
Add os.MkdirAll before creating the routes file.

Fixes #552

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

internal/beads/routes.go

@@ -113,6 +113,11 @@ func RemoveRoute(townRoot string, prefix string) error {
 
 // WriteRoutes writes routes to routes.jsonl, overwriting existing content.
 func WriteRoutes(beadsDir string, routes []Route) error {
+	// Ensure beads directory exists
+	if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		return fmt.Errorf("creating beads directory: %w", err)
+	}
+
 	routesPath := filepath.Join(beadsDir, RoutesFileName)
 
 	file, err := os.Create(routesPath)