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/e2e/test_thinking_live.py

@@ -37,7 +37,7 @@ from polygateway.errors import (
     SourceDeadError,
     TransientError,
 )
-from polygateway.providers import DEFAULT_CAPABILITIES, get_capability
+from polygateway.thinking import DEFAULT_CAPABILITIES, get_capability
 
 _ENV = {k: v for k, v in {**dotenv_values(".env"), **os.environ}.items() if v is not None}
 _HAS_SOURCE = any(k.split("__")[0] == "LLM" and k.endswith("__API_KEY") for k in _ENV)