Fix CI: add psutil to requirements-core.txt

monitoring/metrics.py imports psutil unconditionally; missing from
install requirements caused test_phase4_complete.py collection error.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: RobLe3

requirements-core.txt

@@ -38,4 +38,7 @@ pytest-asyncio>=0.23.0
 hypothesis>=6.92.0
 
 # Async HTTP (used in data/sources/base.py)
-aiohttp>=3.9.0
+aiohttp>=3.9.0
+
+# System monitoring (used in monitoring/metrics.py)
+psutil>=5.9.0