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/__init__.py

@@ -1,7 +1,45 @@
-"""PolyGateway：实验室统一的大语言模型（LLM/VLM/OCR）调度与中转库。
+"""PolyGateway: 实验室统一的大语言模型(LLM/VLM/OCR)调度与中转库。
 
-治理单位是一次模型调用：多源选择、限流、错误分类与重试、熔断、响应缓存、
-流式活性看门狗、遥测（含成本）。架构单一事实源见 research-wiki/ARCHITECTURE.md。
+治理单位是一次模型调用: 多源选择、限流、错误分类与重试、熔断、响应缓存、
+流式活性看门狗、遥测。架构单一事实源见 research-wiki/ARCHITECTURE.md。
+顶层导出即公共 API 面: 错误四分类必须从这里引用(ARCH §5.1 约定②)。
 """
 
+from polygateway.client import GatewayClient, gather_bounded
+from polygateway.config import GatewaySettings
+from polygateway.errors import (
+    AllSourcesExhausted,
+    CircuitOpenError,
+    GatewayUnavailableError,
+    GovernanceBackendError,
+    PolyGatewayError,
+    RequestRejectedError,
+    ResultInvalidError,
+    SourceDeadError,
+    TransientError,
+)
+from polygateway.providers import DEFAULT_PROFILES, ProviderProfile, register_provider
+from polygateway.types import LLMResponse, SourceConfig
+
 __version__ = "0.1.0"
+
+__all__ = [
+    "DEFAULT_PROFILES",
+    "AllSourcesExhausted",
+    "CircuitOpenError",
+    "GatewayClient",
+    "GatewaySettings",
+    "GatewayUnavailableError",
+    "GovernanceBackendError",
+    "LLMResponse",
+    "PolyGatewayError",
+    "ProviderProfile",
+    "RequestRejectedError",
+    "ResultInvalidError",
+    "SourceConfig",
+    "SourceDeadError",
+    "TransientError",
+    "__version__",
+    "gather_bounded",
+    "register_provider",
+]