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_sources.py

@@ -6,8 +6,12 @@ from polygateway.types import SourceConfig, SourceStats
 
 def _src(name):
     return SourceConfig(
-        name=name, provider="openai", base_url="https://gw.example/v1",
-        api_key="sk", model="m", timeout_s=10.0,
+        name=name,
+        provider="openai",
+        base_url="https://gw.example/v1",
+        api_key="sk",
+        model="m",
+        timeout_s=10.0,
     )