Adds wire types and a Provider struct that will replace the
openaicompat-based Ollama shim with a native /api/chat implementation.
Complete and Stream methods are stubs; subsequent commits implement them.
Adjusts the existing ollama.go to drop the type alias on
openaicompat.Provider (renaming the legacy shim to a temporary internal
helper) so the new native Provider type does not collide. Public New()
still returns the openaicompat-backed provider until Task 4 swaps it.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
