feat: add gateway client with env-driven assembly

Includes config aggregation for multi-source env keys, from_env and
from_settings factories with explicit shared-backend injection,
gather_bounded, top-level exports, tightened import-linter layers with
the gate removed from the Makefile, and the finalized .env.example.

tests/unit/test_streaming.py

@@ -47,7 +47,9 @@ class TestThreeLayers:
     async def test_inter_token_timeout(self):
         wrapped = stream_with_liveness_timeouts(
             _emit(["a", "b"], delay_s=0.2, first_delay_s=0.0),
-            ttft_s=1.0, inter_token_s=0.05, total_s=5.0,
+            ttft_s=1.0,
+            inter_token_s=0.05,
+            total_s=5.0,
         )
         with pytest.raises(StreamLivenessTimeout) as ei:
             await _collect(wrapped)