How is that going to find anything that interacts across files?

You misunderstood.

Instead of asking the model: "Here's this codebase, report any vulnerability." you ask. "Here's this codebase, report any vulnerability in module\main.c".

The model can still explore references and other files inside the codebase, but you start over a new context/session for each file in the codebase.

I would think that it is still capable of exploring the codebase and reading other related files like any other coding agent already does.

My phrasing wasn't clear but you aren't telling it to only look at one specific file but to focus its review on one file. Updated my original comment.