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.

src/polygateway/client.py

@@ -26,7 +26,7 @@ from polygateway.middleware.structured import StructuredMW
 from polygateway.middleware.telemetry import TelemetryEmitter, TelemetryMW
 from polygateway.ports import TelemetryStatusProvider
 from polygateway.pricing import PricingTable
-from polygateway.providers import get_capability, get_provider, resolve_thinking
+from polygateway.providers import get_provider
 from polygateway.sources import (
     AdaptivePacer,
     HealthAwareSelector,
@@ -34,6 +34,7 @@ from polygateway.sources import (
     RoundRobinSelector,
     SourceCooldownMemo,
 )
+from polygateway.thinking import get_capability, resolve_thinking
 from polygateway.transports.openai_compat import OpenAICompatTransport
 from polygateway.types import (
     ChatRequest,
@@ -58,7 +59,8 @@ if TYPE_CHECKING:
         TelemetryRecorder,
         Transport,
     )
-    from polygateway.providers import ProviderProfile, ThinkingCapability
+    from polygateway.providers import ProviderProfile
+    from polygateway.thinking import ThinkingCapability
     from polygateway.types import (
         BackpressurePolicy,
         RetryPolicy,