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/examples/sdk/12-full-control.ts

@@ -6,26 +6,22 @@
 
 import { getModel } from "@earendil-works/pi-ai/compat";
 import {
-	AuthStorage,
 	createAgentSession,
 	createExtensionRuntime,
-	ModelRegistry,
+	ModelRuntime,
 	type ResourceLoader,
 	SessionManager,
 	SettingsManager,
 } from "@earendil-works/pi-coding-agent";
 
-// Custom auth storage location
-const authStorage = AuthStorage.create("/tmp/my-agent/auth.json");
-
-// Runtime API key override (not persisted)
+const modelRuntime = await ModelRuntime.create({
+	authPath: "/tmp/my-agent/auth.json",
+	modelsPath: "/tmp/my-agent/models.json",
+});
 if (process.env.MY_ANTHROPIC_KEY) {
-	authStorage.setRuntimeApiKey("anthropic", process.env.MY_ANTHROPIC_KEY);
+	modelRuntime.setRuntimeApiKey("anthropic", process.env.MY_ANTHROPIC_KEY);
 }
 
-// Model registry with no custom models.json
-const modelRegistry = ModelRegistry.inMemory(authStorage);
-
 const model = getModel("anthropic", "claude-sonnet-4-5");
 if (!model) throw new Error("Model not found");
 
@@ -55,8 +51,7 @@ const { session } = await createAgentSession({
 	agentDir: "/tmp/my-agent",
 	model,
 	thinkingLevel: "off",
-	authStorage,
-	modelRegistry,
+	modelRuntime,
 	resourceLoader,
 	tools: ["read", "bash"],
 	sessionManager: SessionManager.inMemory(cwd),