feat(graph): implement FlowGraph class wrapping graphology DirectedGraph

Implements the core FlowGraph class with generic type parameters, DAG-enforced mutations,
cycle detection via graphology-dag, node/edge CRUD, traversal methods, and static factory stubs.

src/graph/index.ts

@@ -1 +1 @@
-export {};
+export { FlowGraph, type FlowGraphOptions } from "./construction.js";