feat: DeerFlow 2.0 core capabilities — Phase 1.0 + 1.1

Phase 1.0 — Butler Mode UI:
- Hide "自动化" and "技能市场" entries from sidebar navigation
- Remove AutomationPanel and SkillMarket view rendering from App.tsx
- Simplify MainViewType to only 'chat'
- Main interface is now: chat + conversation list + detail panel only

Phase 1.1 — Mode Differentiation:
- Add subagent_enabled field to ChatModeConfig (Rust), StreamChatRequest (Tauri),
  gateway-client, kernel-client, saas-relay-client, and streamStore
- TaskTool is now only registered when subagent_enabled=true (Ultra mode)
- System prompt includes sub-agent delegation instructions only in Ultra mode
- Frontend transmits subagent_enabled from ChatMode config through the full stack

This connects the 4-tier mode selector (Flash/Thinking/Pro/Ultra) to actual
backend behavioral differences — Ultra mode now truly enables sub-agent delegation.

desktop/src/lib/kernel-client.ts

@@ -403,7 +403,7 @@ export interface KernelClient {
 
   // Chat (kernel-chat.ts)
   chat(message: string, opts?: { sessionKey?: string; agentId?: string }): Promise<{ runId: string; sessionId?: string; response?: string }>;
-  chatStream(message: string, callbacks: import('./kernel-types').StreamCallbacks, opts?: { sessionKey?: string; agentId?: string; thinking_enabled?: boolean; reasoning_effort?: string; plan_mode?: boolean }): Promise<{ runId: string }>;
+  chatStream(message: string, callbacks: import('./kernel-types').StreamCallbacks, opts?: { sessionKey?: string; agentId?: string; thinking_enabled?: boolean; reasoning_effort?: string; plan_mode?: boolean; subagent_enabled?: boolean }): Promise<{ runId: string }>;
   cancelStream(sessionId: string): Promise<void>;
   fetchDefaultAgentId(): Promise<string | null>;
   setDefaultAgentId(agentId: string): void;