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/__init__.py

@@ -24,6 +24,13 @@ from polygateway.ocr import OcrClient
 from polygateway.pricing import ModelPrice, PricingTable
 from polygateway.providers import DEFAULT_PROFILES, ProviderProfile, register_provider
 from polygateway.telemetry.schema import telemetry_schema_sql
+from polygateway.thinking import (
+    ThinkingCapability,
+    ThinkingUnsupportedError,
+    get_capability,
+    register_capability,
+    resolve_thinking,
+)
 from polygateway.types import (
     EmbeddingResponse,
     LLMResponse,
@@ -32,6 +39,7 @@ from polygateway.types import (
     OcrTextResult,
     SourceConfig,
     TelemetryStatus,
+    ThinkingObservation,
 )
 
 __version__ = "1.3.0"
@@ -63,9 +71,15 @@ __all__ = [
     "SourceDeadError",
     "SourceNotConfiguredError",
     "TelemetryStatus",
+    "ThinkingCapability",
+    "ThinkingObservation",
+    "ThinkingUnsupportedError",
     "TransientError",
     "__version__",
     "gather_bounded",
+    "get_capability",
+    "register_capability",
     "register_provider",
+    "resolve_thinking",
     "telemetry_schema_sql",
 ]