chore(lint): apply ruff --fix (4443 auto-fixes)

Auto-fixed categories:
- F401: unused imports removed
- I001: import blocks sorted
- W293: trailing whitespace stripped
- UP035: deprecated typing imports replaced
- SIM: simplify suggestions applied
- ARG: unused args prefixed with underscore
- T201: print statements removed
- F841: unused variables removed
- RUF059: unpacked variables prefixed

Remaining ~1500 unfixable errors (C901, B904, N806, E402) require manual work.

Backend smoke tests: 13/13 passed.

backend/tests/test_logger.py

@@ -5,18 +5,19 @@
 # @RELATION: VERIFIES -> src/core/logger.py
 # @INVARIANT: All required log statements must correctly check the threshold.
 
-import pytest
 import logging
 
+import pytest
+
+from src.core.config_models import LoggingConfig
 from src.core.logger import (
+    CotJsonFormatter,
     belief_scope,
-    logger,
     configure_logger,
     get_task_log_level,
+    logger,
     should_log_task_level,
-    CotJsonFormatter,
 )
-from src.core.config_models import LoggingConfig
 
 
 @pytest.fixture(autouse=True)
@@ -101,9 +102,8 @@ def test_belief_scope_error_handling(caplog):
 
     caplog.set_level("DEBUG")
 
-    with pytest.raises(ValueError):
-        with belief_scope("FailingFunction"):
-            raise ValueError("Something went wrong")
+    with pytest.raises(ValueError), belief_scope("FailingFunction"):
+        raise ValueError("Something went wrong")
 
     # Check that an EXPLORE marker was emitted
     explore_records = [
@@ -294,7 +294,6 @@ def test_enable_belief_state_flag(caplog):
 def test_cot_json_formatter_output():
     """Test that CotJsonFormatter produces valid JSON with expected fields."""
     import json
-    from datetime import datetime, timezone
 
     formatter = CotJsonFormatter()