Issue 5: PNG generation is a side effect of `build_graph()`

[sanjaigridsandguides]

Problem

• build_graph() writes graph_image.png to disk before returning the compiled graph.
• graph.py calls faq_graph = build_graph() at module top level.
• Every import of graph regenerates the PNG.

Impact

• Adds latency to every run (including unit tests).
• A failure in draw_mermaid_png() would prevent the module from importing at all.

Required Fix

• Move the PNG render to a separate function or to if __name__ == "__main__":.
• Don't run side-effecting I/O inside a constructor function named build_graph.