Add matplotlib to dev dependencies

Required by codeclash.analysis (elo plots + ranking).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: john-b-yang

pyproject.toml

@@ -48,6 +48,7 @@ dev = [
     "Werkzeug",
     "frozen-flask",
     "anthropic",
+    "matplotlib",  # analysis plots + elo ranking (codeclash.analysis)
 ]
 aws = [
     "boto3",