feat: plan agent v2, global chat mode, and user-mode v2 flag sync

- Add runtime/plan_agent_v2 package and shims; gateway/direct_loop/WS wiring

- Admin chat: interaction mode and specialist only in user menu; session API stores memory_mode and execution_mode only

- POST /admin/api/chat/user-mode mirrors plan_agent_version to AIA_EXPERT_PLAN_AGENT_V2_ENABLED (v2 to 1, v1 to 0)

- Composer cleanup (hidden mode select, no reasoning toggle in meta bar); tests and _local/system.env.example

Co-authored-by: Cursor <cursoragent@cursor.com>

interfaces/admin/static/chat.html

@@ -563,7 +563,7 @@
     <!-- Cache-bust for desktop webview: avoid stale chat.js -->
     <script
       defer
-      src="/admin/assets/chat.js?v=20260428-2"
+      src="/admin/assets/chat.js?v=20260501-1"
       onerror="(function(){var s=document.getElementById('chat-boot-splash');if(s){s.querySelector('.chat-boot-splash__title span:last-child').textContent='无法加载 chat.js';s.querySelector('.chat-boot-splash__muted').textContent='请确认网关已启动且 /admin/assets/chat.js 可访问。';}})()"
     ></script>
   </head>