Phase 4: clarify that harness improvements go in the same PR

Signed-off-by: Heng Qian <qianheng@amazon.com>

Author: qianheng-aws

.claude/harness/ppl-bugfix-harness.md

@@ -293,6 +293,8 @@ After each bugfix, check if the harness itself needs updating:
 - **Verification gap** caused rework? → Add step to 3.1 or 2.2 checklist
 - **Representative fix**? → Add row to Case Index below
 
+Include harness improvements in the same PR as the bugfix — they share the same review context.
+
 ---
 
 ## Quick Reference: Symptom → Fix Path