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_errors.py

@@ -34,7 +34,9 @@ class TestBaseShape:
 class TestResultInvalid:
     def test_carries_diagnosis(self):
         exc = ResultInvalidError(
-            "bad json", raw_text="{oops", repair_error="unterminated",
+            "bad json",
+            raw_text="{oops",
+            repair_error="unterminated",
             validation_errors=("field x missing",),
         )
         assert exc.raw_text == "{oops"
@@ -45,7 +47,9 @@ class TestResultInvalid:
 class TestGatewayUnavailable:
     def test_fields_and_inheritance(self):
         exc = AllSourcesExhausted(
-            scope="LLM", reason="retry_exhausted", retry_after_s=4.0,
+            scope="LLM",
+            reason="retry_exhausted",
+            retry_after_s=4.0,
             per_source_reasons={"qwen_1": "timeout"},
         )
         assert isinstance(exc, GatewayUnavailableError)
@@ -66,7 +70,9 @@ class TestGatewayUnavailable:
     def test_per_source_reason_domain_enforced(self):
         with pytest.raises(ValueError):
             AllSourcesExhausted(
-                scope="LLM", reason="no_sources", retry_after_s=0.0,
+                scope="LLM",
+                reason="no_sources",
+                retry_after_s=0.0,
                 per_source_reasons={"qwen_1": "weird"},
             )