fix(agent): reject send tool when targeting the same conversation

Pass replyTarget through the full pipeline (ChatRequest → gateway
identity → agent headers → MCP session) so the send tool can detect
when the destination matches the current conversation and return an
error guiding the agent to reply directly instead.

internal/conversation/types.go

@@ -225,6 +225,7 @@ type ChatRequest struct {
 	RouteID                 string `json:"-"`
 	ChatToken               string `json:"-"`
 	ExternalMessageID       string `json:"-"`
+	ReplyTarget             string `json:"-"`
 	ConversationType        string `json:"-"`
 	ConversationName        string `json:"-"`
 	UserMessagePersisted    bool   `json:"-"`