feat(coding-agent): replace model registry with model runtime

Move provider auth and OAuth flows onto pi-ai Models, compose models.json and extension overlays through ModelRuntime, and retain ModelRegistry as an extension compatibility facade.

packages/coding-agent/test/footer-width.test.ts

@@ -52,7 +52,7 @@ function createSession(options: {
 			getCwd: () => "/tmp/project",
 		},
 		getContextUsage: () => ({ contextWindow: 200_000, percent: 12.3 }),
-		modelRegistry: {
+		modelRuntime: {
 			isUsingOAuth: () => false,
 		},
 	};