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/context.py

@@ -16,6 +16,7 @@ Usage:
     from src.core.context import get_user
     user = get_user()  # Returns current request's user
 """
+
 from contextvars import ContextVar
 
 
@@ -28,6 +29,7 @@ def get_default_user() -> str:
     """
     # Import here to avoid circular dependency
     from src.core.config import config
+
     return config.effective_default_user
 
 
@@ -36,9 +38,7 @@ def get_default_user() -> str:
 # and resolve the real default in get_user()
 _USER_NOT_SET = "__user_not_set__"
 current_user: ContextVar[str] = ContextVar("current_user", default=_USER_NOT_SET)
-current_conversation: ContextVar[str | None] = ContextVar(
-    "current_conversation", default=None
-)
+current_conversation: ContextVar[str | None] = ContextVar("current_conversation", default=None)
 
 
 def apply_tenant_guard(user: str) -> str:
@@ -60,9 +60,8 @@ def apply_tenant_guard(user: str) -> str:
     from src.core.config import PRODUCTION_TENANT, TEST_TENANT, Environment, config
     from src.core.multi_tenancy import sanitize_user_id
 
-    if (
-        config.ENVIRONMENT != Environment.PRODUCTION
-        and sanitize_user_id(user) == sanitize_user_id(PRODUCTION_TENANT)
+    if config.ENVIRONMENT != Environment.PRODUCTION and sanitize_user_id(user) == sanitize_user_id(
+        PRODUCTION_TENANT
     ):
         from src.core.logging_config import get_logger