fix: ignore stale NotebookLM responses

[renanwilliam]

Summary

• wait for existing NotebookLM chat history to hydrate before submitting a new question
• ignore stale and transient response nodes when selecting the newly generated answer
• raise the query timeout to 300 seconds for slower notebooks with many sources

Validation

• ran python3 -m py_compile scripts/ask_question.py scripts/config.py
• reproduced the stale-response behavior locally against a notebook with many sources
• verified the patched script returned the newly generated answer instead of the old intro text

Closes #25.