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/errors.py

@@ -87,7 +87,9 @@ class GatewayUnavailableError(PolyGatewayError):
         reasons = dict(per_source_reasons or {})
         for src, src_reason in reasons.items():
             if src_reason not in SOURCE_REASONS:
-                raise ValueError(f"源 {src!r} 的 reason 非法: {src_reason!r}(允许: {sorted(SOURCE_REASONS)})")
+                raise ValueError(
+                    f"源 {src!r} 的 reason 非法: {src_reason!r}(允许: {sorted(SOURCE_REASONS)})"
+                )
         super().__init__(f"{scope.lower()} 网关暂时不可用: {reason}", source_name=source_name)
         self.scope = scope.lower()
         self.reason = reason