ci: add timeout-minutes to echidna-fuzz rate-limit-check job

The only steps-based job in the workflow set lacking a timeout bound.
Closes the one real `missing_timeout_minutes` gap (the other Hypatia
flags are caller workflows where timeout-minutes is invalid).

Tracker: hyperpolymath/echidna#238

https://claude.ai/code/session_01UAqDQaMwpUqWHUSZekGZWv

Author: claude

.github/workflows/echidna-fuzz.yml

@@ -73,6 +73,7 @@ jobs:
   rate-limit-check:
     name: Rate Limit Check
     runs-on: ubuntu-latest
+    timeout-minutes: 5
     outputs:
       allowed: ${{ steps.check.outputs.allowed }}
       reason: ${{ steps.check.outputs.reason }}