Add Claude Ultracode with setting-change notices (#1625)

* Add Claude Ultracode mode notices

* Share provider notice definitions

* Test Claude Ultracode through public turn API

docs/providers.md

@@ -343,7 +343,7 @@ interface AgentSession {
   getRuntimeInfo(): Promise<AgentRuntimeInfo>;
   getAvailableModes(): Promise<AgentMode[]>;
   getCurrentMode(): Promise<string | null>;
-  setMode(modeId: string): Promise<void>;
+  setMode(modeId: string): Promise<void | AgentProviderNotice>;
   getPendingPermissions(): AgentPermissionRequest[];
   respondToPermission(
     requestId: string,
@@ -355,7 +355,7 @@ interface AgentSession {
   // Optional:
   listCommands?(): Promise<AgentSlashCommand[]>;
   setModel?(modelId: string | null): Promise<void>;
-  setThinkingOption?(thinkingOptionId: string | null): Promise<void>;
+  setThinkingOption?(thinkingOptionId: string | null): Promise<void | AgentProviderNotice>;
   setFeature?(featureId: string, value: unknown): Promise<void>;
   tryHandleOutOfBand?(prompt: AgentPromptInput): {
     run(ctx: { emit: (event: AgentStreamEvent) => void }): Promise<void>;
@@ -363,6 +363,8 @@ interface AgentSession {
 }
 ```
 
+`setMode` and `setThinkingOption` may return an `AgentProviderNotice` when the provider knows the change needs user-facing context. For example, providers that stage changes until the next turn should return an `info` notice while a turn is already running. The app renders the notice generically as a toast; provider-specific lifecycle behavior stays in the provider implementation.
+
 ### Steps
 
 1. Create `packages/server/src/server/agent/providers/{name}-agent.ts` implementing both interfaces