Name the measurement files in the fixture's description

AGENTS.md lists what the explorer's sandbox tree holds and had not caught
up with the three sized source files added beside it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

AGENTS.md

@@ -144,8 +144,10 @@ repo is in PLAN.md's "Backend layout" section.
   home's `~/files` holding the states that are otherwise only reachable by
   finding a real machine in one: an empty directory, a name with a tab in
   it and one with an apostrophe, a binary file, one over `FILE_LIMIT`, one
-  `chmod 000`, a symlink to a directory and a broken one, and a source file
-  per language. Point a session at it with
+  `chmod 000`, a symlink to a directory and a broken one, a source file per
+  language, and the three sizes the limits were measured against
+  (`edit-32k.rs`, `edit-128k.rs`, `big-source.rs`), so those figures can be
+  taken again rather than re-derived. Point a session at it with
   `./ui-sandbox.sh api /sessions/<id>/cwd -X POST -H 'content-type: application/json' -d '{"cwd":"~/files"}'`.
   The 409 is produced by editing the file on the machine (`printf … > file`)
   between pressing the pencil and pressing save.