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/docs/sdk.md

@@ -16,16 +16,12 @@ See [examples/sdk/](../examples/sdk/) for working examples from minimal to full
 ## Quick Start
 
 ```typescript
-import { AuthStorage, createAgentSession, ModelRegistry, SessionManager } from "@earendil-works/pi-coding-agent";
-
-// Set up credential storage and model registry
-const authStorage = AuthStorage.create();
-const modelRegistry = ModelRegistry.create(authStorage);
+import { createAgentSession, ModelRuntime, SessionManager } from "@earendil-works/pi-coding-agent";
 
+const modelRuntime = await ModelRuntime.create();
 const { session } = await createAgentSession({
   sessionManager: SessionManager.inMemory(),
-  authStorage,
-  modelRegistry,
+  modelRuntime,
 });
 
 session.subscribe((event) => {
@@ -369,10 +365,9 @@ When you pass a custom `ResourceLoader`, `cwd` and `agentDir` no longer control
 
 ```typescript
 import { getModel } from "@earendil-works/pi-ai";
-import { AuthStorage, ModelRegistry } from "@earendil-works/pi-coding-agent";
+import { ModelRuntime } from "@earendil-works/pi-coding-agent";
 
-const authStorage = AuthStorage.create();
-const modelRegistry = ModelRegistry.create(authStorage);
+const modelRuntime = await ModelRuntime.create();
 
 // Find specific built-in model (doesn't check if API key exists)
 const opus = getModel("anthropic", "claude-opus-4-5");
@@ -380,10 +375,10 @@ if (!opus) throw new Error("Model not found");
 
 // Find any model by provider/id, including custom models from models.json
 // (doesn't check if API key exists)
-const customModel = modelRegistry.find("my-provider", "my-model");
+const customModel = modelRuntime.getModel("my-provider", "my-model");
 
-// Get only models that have valid API keys configured
-const available = await modelRegistry.getAvailable();
+// Get only models that have valid authentication configured
+const available = await modelRuntime.getAvailable();
 
 const { session } = await createAgentSession({
   model: opus,
@@ -395,8 +390,7 @@ const { session } = await createAgentSession({
     { model: haiku, thinkingLevel: "off" },
   ],
   
-  authStorage,
-  modelRegistry,
+  modelRuntime,
 });
 ```
 
@@ -415,14 +409,14 @@ import {
 
 const cliModel = resolveCliModel({
   cliModel: "anthropic/claude-opus-4-5:high",
-  modelRegistry,
+  modelRuntime,
 });
 if (cliModel.error) throw new Error(cliModel.error);
 if (cliModel.warning) console.warn(cliModel.warning);
 
 const { scopedModels, diagnostics } = await resolveModelScopeWithDiagnostics(
   ["anthropic/*:high", "gpt-5"],
-  modelRegistry,
+  modelRuntime,
 );
 for (const diagnostic of diagnostics) {
   console.warn(diagnostic.message);
@@ -435,40 +429,41 @@ for (const diagnostic of diagnostics) {
 
 ### API Keys and OAuth
 
-API key resolution priority (handled by AuthStorage):
+Authentication resolution priority (handled by `ModelRuntime`):
 1. Runtime overrides (via `setRuntimeApiKey`, not persisted)
 2. Stored credentials in `auth.json` (API keys or OAuth tokens)
 3. Environment variables (`ANTHROPIC_API_KEY`, `OPENAI_API_KEY`, etc.)
 4. Fallback resolver (for custom provider keys from `models.json`)
 
 ```typescript
-import { AuthStorage, ModelRegistry } from "@earendil-works/pi-coding-agent";
+import { InMemoryCredentialStore } from "@earendil-works/pi-ai";
+import { createAgentSession, ModelRuntime } from "@earendil-works/pi-coding-agent";
 
 // Default: uses ~/.pi/agent/auth.json and ~/.pi/agent/models.json
-const authStorage = AuthStorage.create();
-const modelRegistry = ModelRegistry.create(authStorage);
+const modelRuntime = await ModelRuntime.create();
 
-const { session } = await createAgentSession({
-  sessionManager: SessionManager.inMemory(),
-  authStorage,
-  modelRegistry,
-});
+// Provider-owned auth methods and current status
+for (const provider of modelRuntime.getProviders()) {
+  const status = await modelRuntime.checkAuth(provider.id);
+  console.log(provider.name, provider.auth, status);
+}
 
 // Runtime API key override (not persisted to disk)
-authStorage.setRuntimeApiKey("anthropic", "sk-my-temp-key");
+modelRuntime.setRuntimeApiKey("anthropic", "sk-my-temp-key");
 
-// Custom auth storage location
-const customAuth = AuthStorage.create("/my/app/auth.json");
-const customRegistry = ModelRegistry.create(customAuth, "/my/app/models.json");
-
-const { session } = await createAgentSession({
-  sessionManager: SessionManager.inMemory(),
-  authStorage: customAuth,
-  modelRegistry: customRegistry,
+// Custom credential and model locations
+const customRuntime = await ModelRuntime.create({
+  authPath: "/my/app/auth.json",
+  modelsPath: "/my/app/models.json",
 });
 
-// No custom models.json (built-in models only)
-const simpleRegistry = ModelRegistry.inMemory(authStorage);
+// Or inject any pi-ai CredentialStore
+const credentials = new InMemoryCredentialStore();
+const inMemoryRuntime = await ModelRuntime.create({ credentials });
+
+const { session } = await createAgentSession({
+  modelRuntime: customRuntime,
+});
 ```
 
 > See [examples/sdk/09-api-keys-and-oauth.ts](../examples/sdk/09-api-keys-and-oauth.ts)
@@ -927,26 +922,22 @@ interface LoadExtensionsResult {
 import { getModel } from "@earendil-works/pi-ai";
 import { Type } from "typebox";
 import {
-  AuthStorage,
   createAgentSession,
   DefaultResourceLoader,
   defineTool,
-  ModelRegistry,
+  ModelRuntime,
   SessionManager,
   SettingsManager,
 } from "@earendil-works/pi-coding-agent";
 
-// Set up auth storage (custom location)
-const authStorage = AuthStorage.create("/custom/agent/auth.json");
-
-// Runtime API key override (not persisted)
+const modelRuntime = await ModelRuntime.create({
+  authPath: "/custom/agent/auth.json",
+  modelsPath: "/custom/agent/models.json",
+});
 if (process.env.MY_KEY) {
-  authStorage.setRuntimeApiKey("anthropic", process.env.MY_KEY);
+  modelRuntime.setRuntimeApiKey("anthropic", process.env.MY_KEY);
 }
 
-// Model registry (no custom models.json)
-const modelRegistry = ModelRegistry.create(authStorage);
-
 // Inline tool
 const statusTool = defineTool({
   name: "status",
@@ -982,8 +973,7 @@ const { session } = await createAgentSession({
 
   model,
   thinkingLevel: "off",
-  authStorage,
-  modelRegistry,
+  modelRuntime,
 
   tools: ["read", "bash", "status"],
   customTools: [statusTool],
@@ -1149,8 +1139,8 @@ createAgentSessionRuntime
 AgentSessionRuntime
 
 // Auth and Models
-AuthStorage
-ModelRegistry
+ModelRuntime // implements pi-ai Models and owns credential storage
+ModelRegistry // synchronous extension compatibility facade
 resolveCliModel
 resolveModelScopeWithDiagnostics