fix: address independent verification findings

Classify empty completions as transient per human ruling (fixes flaky
real-gateway smoke and prevents caching empty responses), rename the
factory injection parameter gate to breaker per the frozen design,
rewrite the probe-entry cleanup without except BaseException, declare
python-dotenv explicitly, add a mid-backoff cancellation test, and
record all implementation errata in the design and architecture docs.

tests/integration/test_governance_stack.py

@@ -65,7 +65,7 @@ def _full_client(handler, *, clock=None, telemetry=None, cache=None):
             global_limits=GlobalLimits(0, 0, 0),
             now=clock,
         ),
-        gate=InMemoryGate(config=_BREAKER, now=clock),
+        breaker=InMemoryGate(config=_BREAKER, now=clock),
         transport=OpenAICompatTransport(
             client_factory=lambda s: httpx.AsyncClient(transport=httpx.MockTransport(handler))
         ),