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/contracts/test_service_contracts.py

@@ -11,6 +11,7 @@ Semantics:
 
 Run with: make test-contracts
 """
+
 import json
 
 import httpx
@@ -66,9 +67,9 @@ class TestOllamaContract:
         assert response.status_code == 200
         names = [m["name"] for m in response.json()["models"]]
         model = config.OLLAMA_DEFAULT_MODEL
-        assert model in names or f"{model}:latest" in names, (
-            f"{model} not pulled; available: {names}"
-        )
+        assert (
+            model in names or f"{model}:latest" in names
+        ), f"{model} not pulled; available: {names}"
 
     async def test_generate_returns_plain_text(self):
         # Mirrors StewardAgent._call_ollama()
@@ -94,9 +95,7 @@ class TestOllamaContract:
             "ollama",
             {
                 "model": config.OLLAMA_DEFAULT_MODEL,
-                "messages": [
-                    {"role": "user", "content": "What is 6 * 7? Use the calculator."}
-                ],
+                "messages": [{"role": "user", "content": "What is 6 * 7? Use the calculator."}],
                 "tools": [CALCULATOR_TOOL],
                 "tool_choice": "required",
                 "stream": False,