In some evals, the teacher produced hallucinations, seemingly due to
context rot. This makes the zeta prompt crate's budgeted rendering
usable by the teacher, so that it can truncate the list of excerpts.
I've also cleaned up the implementation of zeta_prompt's
`format_related_files_within_budget`, and changed the behavior so that
it filters the the excerpts by priority but renders the files in their
original order.
Release Notes:
- N/A
