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_structured.py

@@ -21,9 +21,19 @@ class Verdict(BaseModel):
 
 def _resp(content):
     return LLMResponse(
-        content=content, thinking="", model="m", provider="p", prompt_tokens=1,
-        completion_tokens=2, latency_ms=10, ttft_ms=None, max_inter_token_ms=None,
-        cache_hit=False, call_id="cid", source_name="s1", usage_source="measured",
+        content=content,
+        thinking="",
+        model="m",
+        provider="p",
+        prompt_tokens=1,
+        completion_tokens=2,
+        latency_ms=10,
+        ttft_ms=None,
+        max_inter_token_ms=None,
+        cache_hit=False,
+        call_id="cid",
+        source_name="s1",
+        usage_source="measured",
     )
 
 
@@ -43,10 +53,10 @@ class TestJsonRepairStrategy:
     @pytest.mark.parametrize(
         "dirty",
         [
-            '```json\n{"answer": 1, "reason": "ok"}\n```',   # 围栏
-            '{"answer": 1, "reason": "ok",}',                  # 尾逗号
-            "{'answer': 1, 'reason': 'ok'}",                   # 单引号
-            '{"answer": 1, "reason": "ok"',                    # 缺右括号
+            '```json\n{"answer": 1, "reason": "ok"}\n```',  # 围栏
+            '{"answer": 1, "reason": "ok",}',  # 尾逗号
+            "{'answer': 1, 'reason': 'ok'}",  # 单引号
+            '{"answer": 1, "reason": "ok"',  # 缺右括号
         ],
     )
     def test_repairs_real_world_dirt(self, dirty):
@@ -79,7 +89,7 @@ class TestNativeSchemaStrategy:
 
 
 def _mw(**kwargs):
-    defaults = dict(strategy=JsonRepairStrategy(), max_retries=1, escalation=None)
+    defaults = {"strategy": JsonRepairStrategy(), "max_retries": 1, "escalation": None}
     defaults.update(kwargs)
     return StructuredMW(**defaults)