Add formatting-first step to existing repo checklist

docs/EXISTING_REPO_CHECKLIST.md

@@ -6,6 +6,13 @@ Use this checklist when beginning work in a repo that may not yet conform to our
 Work on a feature branch. Check each item and fix any gaps before proceeding
 with your task.
 
+## Formatting (do this first)
+
+- [ ] If the repo has never been formatted to our standards, run `make fmt` and
+      commit the result as a standalone branch/commit/PR before any other
+      changes. Formatting diffs can be large and should not be mixed with
+      functional changes.
+
 ## Required Files
 
 - [ ] `README.md` exists with all required sections (Description, Getting