refactor(oauth): add provider registry

packages/coding-agent/docs/extensions.md

@@ -1146,6 +1146,67 @@ pi.events.on("my:event", (data) => { ... });
 pi.events.emit("my:event", { ... });
 ```
 
+### pi.registerProvider(name, config)
+
+Register or override a model provider dynamically. Useful for proxies, custom endpoints, or team-wide model configurations.
+
+```typescript
+// Register a new provider with custom models
+pi.registerProvider("my-proxy", {
+  baseUrl: "https://proxy.example.com",
+  apiKey: "PROXY_API_KEY",  // env var name or literal
+  api: "anthropic-messages",
+  models: [
+    {
+      id: "claude-sonnet-4-20250514",
+      name: "Claude 4 Sonnet (proxy)",
+      reasoning: false,
+      input: ["text", "image"],
+      cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0 },
+      contextWindow: 200000,
+      maxTokens: 16384
+    }
+  ]
+});
+
+// Override baseUrl for an existing provider (keeps all models)
+pi.registerProvider("anthropic", {
+  baseUrl: "https://proxy.example.com"
+});
+
+// Register provider with OAuth support for /login
+pi.registerProvider("corporate-ai", {
+  baseUrl: "https://ai.corp.com",
+  api: "openai-responses",
+  models: [...],
+  oauth: {
+    name: "Corporate AI (SSO)",
+    async login(callbacks) {
+      // Custom OAuth flow
+      callbacks.onAuth({ url: "https://sso.corp.com/..." });
+      const code = await callbacks.onPrompt({ message: "Enter code:" });
+      return { refresh: code, access: code, expires: Date.now() + 3600000 };
+    },
+    async refreshToken(credentials) {
+      // Refresh logic
+      return credentials;
+    },
+    getApiKey(credentials) {
+      return credentials.access;
+    }
+  }
+});
+```
+
+**Config options:**
+- `baseUrl` - API endpoint URL. Required when defining models.
+- `apiKey` - API key or environment variable name. Required when defining models (unless `oauth` provided).
+- `api` - API type: `"anthropic-messages"`, `"openai-completions"`, `"openai-responses"`, etc.
+- `headers` - Custom headers to include in requests.
+- `authHeader` - If true, adds `Authorization: Bearer` header automatically.
+- `models` - Array of model definitions. If provided, replaces all existing models for this provider.
+- `oauth` - OAuth provider config for `/login` support. When provided, the provider appears in the login menu.
+
 ## State Management
 
 Extensions with state should store it in tool result `details` for proper branching support: