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.

src/polygateway/types.py

@@ -112,7 +112,9 @@ class SourceConfig:
             if not getattr(self, attr).strip():
                 raise ValueError(f"SourceConfig.{attr} 不能为空")
         if self.missing_done not in _MISSING_DONE_DOMAIN:
-            raise ValueError(f"missing_done 必须是 {sorted(_MISSING_DONE_DOMAIN)}: {self.missing_done!r}")
+            raise ValueError(
+                f"missing_done 必须是 {sorted(_MISSING_DONE_DOMAIN)}: {self.missing_done!r}"
+            )
 
     def _validate_gates(self) -> None:
         if self.timeout_s <= 0: