feat(gateway): 消息mode落库(A3) — 持久化真实写入渲染模式

- POST /api/v1/chat 与 ws.chat 上行新增可选 mode（行为模式 design/01 §3/design/16 §5.1）
- Web路径经ctx透传本轮mode到编排器持久化(persistUserMessages/persistAssistantReply)，缺省回落chat
- pendingMsg记录mode，恢复/续接路径沿用发起轮次mode
- IM路径(persistToMain/persistIsolated)恒为chat
- 新增/扩充测试：REST/WS/orchestrator/channelbridge 覆盖mode写入
- Docs/api-client.md v0.20 同步mode字段

internal/services/gateway/rest.go

@@ -72,11 +72,15 @@ type loginResponse struct {
 // loop routes it through the vision/OCR tool or the main model (design/06 §0).
 // DeviceID is the client-produced identifier of the sending browser/device
 // (design/16 §4.5), e.g. "web:chrome:a1b2c3"; it is optional and persisted as-is.
+// Mode is the behavior mode the client sent the turn in (design/01 §3, design/16
+// §5.1); it is optional and persisted per message for bubble rendering, falling
+// back to "chat" when omitted.
 type chatRequest struct {
 	SessionID string `json:"sessionId"`
 	Text      string `json:"text"`
 	Image     string `json:"image,omitempty"`
 	DeviceID  string `json:"deviceId,omitempty"`
+	Mode      string `json:"mode,omitempty"`
 	Stream    bool   `json:"stream"`
 }
 
@@ -129,7 +133,7 @@ func (s *Server) handleChat(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	if req.Stream {
-		s.publishChatStream(r.Context(), req.SessionID, req.Text, req.Image, req.DeviceID)
+		s.publishChatStream(r.Context(), req.SessionID, req.Text, req.Image, req.DeviceID, req.Mode)
 		writeJSON(w, http.StatusOK, map[string]any{"stream": true})
 		return
 	}
@@ -144,7 +148,7 @@ func (s *Server) handleChat(w http.ResponseWriter, r *http.Request) {
 	// it binds the full-access env: memory read+write, unchanged behavior.
 	msgs := userChatMessages(req.Text, req.Image)
 	if s.hub.orchestrator != nil {
-		sr, res := s.hub.orchestrator.SubmitSync(withDeviceID(r.Context(), req.DeviceID), req.SessionID, msgs, loop.OwnerEnv())
+		sr, res := s.hub.orchestrator.SubmitSync(withMode(withDeviceID(r.Context(), req.DeviceID), req.Mode), req.SessionID, msgs, loop.OwnerEnv())
 		if !sr.Accepted {
 			if sr.PendingFull {
 				writeError(w, apperr.Wrap(apperr.ErrConflict, "chat_busy", "会话正忙，待处理消息已满"))