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/index.ts

@@ -1 +1,3 @@
-export * from "./error/index.js";
+export * from "./error/index.js";
+
+export { FlowGraph, type FlowGraphOptions } from "./graph/index.js";