Document plain PR titles

Author: MaxGhenis

CLAUDE.md

@@ -29,6 +29,7 @@
 - CI requires access to secrets that are not available to fork PRs for security reasons
 - Fork PRs will fail on data download steps and cannot be merged
 - Before opening a PR, always run `make push-pr-branch` from the repo root. This pushes the current branch to the `upstream` remote and sets the upstream tracking branch correctly for PR creation.
+- Do not prefix PR titles with `[codex]` or any other agent label. Use the plain descriptive title.
 - Always create branches directly on the upstream repository:
   ```bash
   git checkout main