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/02-custom-model.ts

@@ -5,11 +5,9 @@
  */
 
 import { getModel } from "@earendil-works/pi-ai/compat";
-import { AuthStorage, createAgentSession, ModelRegistry } from "@earendil-works/pi-coding-agent";
+import { createAgentSession, ModelRuntime } from "@earendil-works/pi-coding-agent";
 
-// Set up auth storage and model registry
-const authStorage = AuthStorage.create();
-const modelRegistry = ModelRegistry.create(authStorage);
+const modelRuntime = await ModelRuntime.create();
 
 // Option 1: Find a specific built-in model by provider/id
 const opus = getModel("anthropic", "claude-opus-4-5");
@@ -18,13 +16,13 @@ if (opus) {
 }
 
 // Option 2: Find model via registry (includes custom models from models.json)
-const customModel = modelRegistry.find("my-provider", "my-model");
+const customModel = modelRuntime.getModel("my-provider", "my-model");
 if (customModel) {
 	console.log(`Found custom model: ${customModel.provider}/${customModel.id}`);
 }
 
 // Option 3: Pick from available models (have valid API keys)
-const available = await modelRegistry.getAvailable();
+const available = await modelRuntime.getAvailable();
 console.log(
 	"Available models:",
 	available.map((m) => `${m.provider}/${m.id}`),
@@ -34,8 +32,7 @@ if (available.length > 0) {
 	const { session } = await createAgentSession({
 		model: available[0],
 		thinkingLevel: "medium", // off, low, medium, high
-		authStorage,
-		modelRegistry,
+		modelRuntime,
 	});
 
 	try {