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>

src/core/models.py

@@ -2,6 +2,7 @@
 Custom Pydantic base models for consistent serialization.
 Following best practice of having a global base model.
 """
+
 from datetime import datetime
 from typing import Any
 
@@ -17,12 +18,13 @@ def datetime_to_iso_str(dt: datetime) -> str:
 class CustomBaseModel(BaseModel):
     """
     Custom base model with consistent configuration.
-    
+
     All domain models should inherit from this for:
     - Consistent JSON serialization
     - Timezone-aware datetime handling
     - Alias population support
     """
+
     model_config = ConfigDict(
         json_encoders={datetime: datetime_to_iso_str},
         populate_by_name=True,
@@ -30,14 +32,13 @@ class CustomBaseModel(BaseModel):
         validate_assignment=True,
         arbitrary_types_allowed=True,
     )
-    
+
     def serializable_dict(self, **kwargs: Any) -> dict[str, Any]:
         """
         Return dict with only JSON-serializable fields.
-        
+
         Useful for logging and debugging.
         """
         return jsonable_encoder(
-            self.model_dump(**kwargs),
-            custom_encoder={datetime: datetime_to_iso_str}
+            self.model_dump(**kwargs), custom_encoder={datetime: datetime_to_iso_str}
         )