refactor: give reasoning decisions their own module

providers.py had been holding two jobs: the registry of what each
provider looks like, and the decisions made from those declarations.
Adding response-side judgement would have made it the module for
everything about reasoning, so the decisions move to thinking.py and
the registry keeps only profiles and their lookup.

Moving a module breaks any deep-path import of what moved, so the six
public symbols are promoted to the package root at the same time. The
top level is this library's stated API surface; giving downstream a
stable name to import is what makes the next reorganisation harmless.
observe_thinking stays unexported — downstream reads the verdict off
LLMResponse, and exporting it would be a permanent promise for nothing.

tests/unit/test_package.py

@@ -49,3 +49,28 @@ def test_telemetry_status_exported():
     assert "TelemetryStatus" in polygateway.__all__
     assert polygateway.TelemetryStatus is not None
     assert "TelemetryStatusProvider" not in polygateway.__all__
+
+
+def test_thinking_public_surface_exported():
+    """issue #16/#17: 推理决策搬进 `polygateway.thinking` 后,公共符号必须走顶层。
+
+    搬模块本身会断掉 `from polygateway.providers import ThinkingCapability` 这类
+    深路径 import。给下游一个稳定引用点,是以后再重组不再破坏下游的前提——本库
+    的约定是「顶层导出即公共 API 面」。
+
+    `observe_thinking` / `reconcile_thinking` **不**导出: 它们是 transport 内部
+    的裁定与对账,下游读 `LLMResponse.thinking_observation` 即可,导出即多一份
+    永久承诺。
+    """
+    for name in (
+        "ThinkingCapability",
+        "ThinkingObservation",
+        "ThinkingUnsupportedError",
+        "get_capability",
+        "register_capability",
+        "resolve_thinking",
+    ):
+        assert hasattr(polygateway, name), name
+        assert name in polygateway.__all__, name
+    assert "observe_thinking" not in polygateway.__all__
+    assert "reconcile_thinking" not in polygateway.__all__