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>

runtime/plan_agent_v2/switch.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+import os
+from typing import Any
+
+
+def _is_truthy(raw: str | None) -> bool:
+    return str(raw or "").strip().lower() in {"1", "true", "yes", "on"}
+
+
+def v2_feature_enabled(*, store: Any | None = None) -> bool:
+    # Default off for shadow path safety.
+    raw = ""
+    try:
+        if store is not None:
+            raw = str(store.get_setting("AIA_EXPERT_PLAN_AGENT_V2_ENABLED") or "").strip()
+    except Exception:
+        raw = ""
+    if not raw:
+        raw = str(os.getenv("AIA_EXPERT_PLAN_AGENT_V2_ENABLED") or "").strip()
+    if not raw:
+        return False
+    return _is_truthy(raw)
+
+
+def should_route_to_v2(*, store: Any | None, interaction_mode: str, force_flag: bool = False) -> bool:
+    if str(interaction_mode or "").strip().lower() != "expert":
+        return False
+    if force_flag:
+        return True
+    return v2_feature_enabled(store=store)
+
+
+__all__ = ["should_route_to_v2", "v2_feature_enabled"]
+