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/agent-session-runtime-events.test.ts

@@ -10,6 +10,7 @@ import {
 	createAgentSessionServices,
 } from "../src/core/agent-session-runtime.ts";
 import { AuthStorage } from "../src/core/auth-storage.ts";
+import { ModelRuntime } from "../src/core/model-runtime.ts";
 import { SessionManager } from "../src/core/session-manager.ts";
 import type {
 	ExtensionFactory,
@@ -42,11 +43,33 @@ describe("AgentSessionRuntime session lifecycle events", () => {
 		faux.setResponses([fauxAssistantMessage("one"), fauxAssistantMessage("two"), fauxAssistantMessage("three")]);
 
 		const authStorage = AuthStorage.inMemory();
-		authStorage.setRuntimeApiKey(faux.getModel().provider, "faux-key");
+		await authStorage.modify(faux.getModel().provider, async () => ({ type: "api_key", key: "faux-key" }));
+		const modelRuntime = await ModelRuntime.create({
+			credentials: authStorage,
+			modelsPath: join(tempDir, "models.json"),
+		});
+		const model = faux.getModel();
+		modelRuntime.registerProvider(model.provider, {
+			baseUrl: model.baseUrl,
+			api: model.api,
+			models: [
+				{
+					id: model.id,
+					name: model.name,
+					api: model.api,
+					reasoning: model.reasoning,
+					input: model.input,
+					cost: model.cost,
+					contextWindow: model.contextWindow,
+					maxTokens: model.maxTokens,
+					baseUrl: model.baseUrl,
+				},
+			],
+		});
 
 		const runtimeOptions = {
 			agentDir: tempDir,
-			authStorage,
+			modelRuntime,
 			model: faux.getModel(),
 			resourceLoaderOptions: {
 				extensionFactories: [extensionFactory],