style: apply ruff's automatic fixes and formatter

Mechanical only, and separated from the judgment calls that follow so the
reviewable changes are not buried in a 98-file whitespace diff.

227 automatic fixes: 60 blank lines carrying whitespace, 60 unsorted import
blocks, 34 Optional[X] to X | None, 28 unused imports, 16 deprecated typing
imports, 12 datetime.timezone.utc to datetime.UTC, and assorted smaller
modernisations. Then `ruff format` over src and tests: 98 files reformatted,
35 already conforming.

No file among the unused-import findings defines __all__ or is an __init__.py,
so nothing here removes a re-export.

`make test`: 658 passed, unchanged from HEAD.

Two things observed while verifying, neither addressed here:

`pytest tests/` cannot collect — tests/e2e/test_orchestration_e2e.py uses an
`e2e` marker that is not registered, and the config is strict about markers.
This fails identically at HEAD, so it predates this change; `make test` passes
because it ignores tests/e2e, tests/integration and tests/contracts.

test_tatlock_tool_call_logging_calculator is flaky. It failed once in a full run
with these changes and passed on the next, passes in isolation with them, and
fails in isolation at HEAD. It is order- or timing-dependent, not a regression
from this commit — established by running the full suite both ways rather than
by reasoning about which change could have caused it.

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

tests/agents/test_tools.py

@@ -5,21 +5,20 @@ Note: Web search has been moved to The Librarian agent.
 See tests/agents/librarian/test_tools.py for search tests.
 """
 
-import pytest
 from datetime import datetime
 
 from src.agents.tools import (
     calculate,
-    get_current_datetime,
     calculate_time_offset,
+    get_current_datetime,
     time_difference,
 )
 
-
 # ============================================================================
 # Calculator Tests
 # ============================================================================
 
+
 class TestCalculator:
     """Tests for the calculator tool."""
 
@@ -81,6 +80,7 @@ class TestCalculator:
 # Date/Time Tests
 # ============================================================================
 
+
 class TestDateTime:
     """Tests for date/time toolkit."""