[FR] Improve support for asking questions over a large number of repositories

[brendan-kellam]

Problem

Sometimes, users want to ask questions that cover the entire breadth of the corpus, e.g., "what repositories contain X, Y, and Z", or "show me examples of foo and bar". For these types of questions, our repository selector dropdown is a bit of a obtuse UX, especially when you have lots (1k+) repos. What repositories should you select? Pressing "select all" doesn't help either as the agent isn't well equipped to navigate over such a large number of repos.

Proposed solution

Ideally, users shouldn't even need to make a selection at all - the agent is intelligently able to filter down the set of repositories that it should search while efficiently managing its context window.