Add bug fix diff structure guideline to CLAUDE.md

Summary:
Document the two-diff workflow for bug fixes: first diff adds a test
asserting the broken behavior (with FIXME), second diff fixes the bug
and updates the test.

Adding because this stack is so much easier to review with this approach, and claude actually struggled to do this on it's own and needed redicerction a couple of times

Reviewed By: Joald

Differential Revision: D99674045

fbshipit-source-id: ddb55b05a93c75db96391b2e5bf618061ff14cdb

Author: Clara Rull

eden/mononoke/.llms/rules/bug_fix_diff_structure.md

@@ -0,0 +1,28 @@
+---
+oncalls: ['scm_server_infra']
+apply_to_user_prompt: 'fix.{0,20}bug|bug.{0,20}fix|bugfix'
+---
+
+# Bug Fix Diff Structure
+
+**Severity: MEDIUM**
+
+## What to Look For
+
+- A non-trivial bug fix that changes code and tests in the same diff without first demonstrating the broken behavior
+
+## Do NOT Flag
+
+- Small, obvious fixes where the correctness is clear from reading the diff (typos, one-liners, straightforward few-line changes)
+- Fixes where existing tests already cover the broken behavior and just need assertion updates
+- When already working on a diff stack that follows this pattern
+- Fixes to config, logging, metrics, or other areas where the bug is not reproducible in unit or integration tests
+
+## Recommendation
+
+For non-trivial bug fixes where the behavior is testable, consider a two-diff stack:
+
+1. **First diff**: Add a test asserting the current broken behavior with a `FIXME` comment.
+2. **Second diff**: Fix the bug and update the test, removing the `FIXME`.
+
+Both diffs should pass tests independently. This makes the bug visible in review and proves the fix actually changes behavior.