Refactor OAuth/API key handling: AuthStorage and ModelRegistry

- Add AuthStorage class for credential storage (auth.json)
- Add ModelRegistry class for model management with API key resolution
- Add discoverAuthStorage() and discoverModels() discovery functions
- Add migration from legacy oauth.json and settings.json apiKeys to auth.json
- Remove configureOAuthStorage, defaultGetApiKey, findModel, discoverAvailableModels
- Remove apiKeys from Settings type and SettingsManager methods
- Rename getOAuthPath to getAuthPath
- Update SDK, examples, docs, tests, and mom package

Fixes #296

packages/coding-agent/examples/sdk/02-custom-model.ts

@@ -4,16 +4,27 @@
  * Shows how to select a specific model and thinking level.
  */
 
-import { createAgentSession, discoverAvailableModels, findModel } from "../../src/index.js";
+import { getModel } from "@mariozechner/pi-ai";
+import { createAgentSession, discoverAuthStorage, discoverModels } from "../../src/index.js";
 
-// Option 1: Find a specific model by provider/id
-const { model: sonnet } = findModel("anthropic", "claude-sonnet-4-20250514");
-if (sonnet) {
-	console.log(`Found model: ${sonnet.provider}/${sonnet.id}`);
+// Set up auth storage and model registry
+const authStorage = discoverAuthStorage();
+const modelRegistry = discoverModels(authStorage);
+
+// Option 1: Find a specific built-in model by provider/id
+const opus = getModel("anthropic", "claude-opus-4-5");
+if (opus) {
+	console.log(`Found model: ${opus.provider}/${opus.id}`);
 }
 
-// Option 2: Pick from available models (have valid API keys)
-const available = await discoverAvailableModels();
+// Option 2: Find model via registry (includes custom models from models.json)
+const customModel = modelRegistry.find("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();
 console.log(
 	"Available models:",
 	available.map((m) => `${m.provider}/${m.id}`),
@@ -23,6 +34,8 @@ if (available.length > 0) {
 	const { session } = await createAgentSession({
 		model: available[0],
 		thinkingLevel: "medium", // off, low, medium, high
+		authStorage,
+		modelRegistry,
 	});
 
 	session.subscribe((event) => {