talk query enhancement

[davidguttman]

We want to enhance the AI in Action bot to enable users to query information about previous talks. The bot should be able to respond to questions like "@bot has there been a talk about A2A before?" or "@bot what talks have been about agents?". Response should follow same format as responses to questions about upcoming speakers.

This will require planning the architecture and implementation. Please use a modular approach to facilitate independent testing of components.

Design the high-level architecture for this feature, considering how to store and retrieve information about past talks.
Break down the implementation into smaller, manageable steps.
Specify which files need to be updated or added, including mongo db for storing talk information and for handling user queries.
Outline how the modular design will support independent testing of each component.
this is a low usage app, don’t need to rely on the db for querying specific talks, just pull all and let LLM evaluate matches

Before making any changes, please provide a planning doc detailing the architecture, steps, and file modifications.

PR to close this issue should contain:

1. Planning doc docs/plans/03-talk-history.md that specifies the required tests and implementation
2. New tests from planning doc
3. New implementation changes from planning doc