refactor(session): 移除 InboundMessage.dialog_id，dialog_id 完全由 SessionManager 管理

Channel 消息不再携带 dialog_id，完全由 SessionManager 通过
current_sessions 内部管理。/new 后下一条消息会自动路由到新 session。

改动：
- InboundMessage 移除 dialog_id 字段
- handle_message 移除 dialog_id 参数
- Feishu/CLI channel 创建 InboundMessage 时不再设置 dialog_id
- 路由逻辑简化为：current_sessions → find_active_session → 创建新 session

src/bus/message.rs

@@ -164,7 +164,6 @@ pub struct InboundMessage {
     pub channel: String,
     pub sender_id: String,
     pub chat_id: String,
-    pub dialog_id: Option<String>,
     pub content: String,
     pub timestamp: i64,
     pub media: Vec<MediaItem>,

src/channels/cli_chat.rs

@@ -117,7 +117,6 @@ impl CliChatChannel {
                     channel: self.name().to_string(),
                     sender_id: "cli".to_string(),
                     chat_id: chat_id.unwrap_or_else(short_id),
-                    dialog_id: None,
                     content,
                     timestamp: crate::bus::message::current_timestamp(),
                     media: Vec::new(),

src/channels/feishu.rs

@@ -1110,7 +1110,6 @@ impl FeishuChannel {
                                                 channel: "feishu".to_string(),
                                                 sender_id: parsed.open_id.clone(),
                                                 chat_id: parsed.chat_id.clone(),
-                                                dialog_id: None, // Use default/current dialog
                                                 content: parsed.content.clone(),
                                                 timestamp: crate::bus::message::current_timestamp(),
                                                 media: parsed.media.map(|m| vec![m]).unwrap_or_default(),

src/gateway/mod.rs

@@ -107,7 +107,6 @@ impl GatewayState {
                             &inbound.channel,
                             &inbound.sender_id,
                             &inbound.chat_id,
-                            inbound.dialog_id.as_deref(),
                             &inbound.content,
                             inbound.media,
                         ).await {

src/session/session.rs

@@ -1039,15 +1039,11 @@ impl SessionManager {
         channel: &str,
         _sender_id: &str,
         chat_id: &str,
-        dialog_id: Option<&str>,
         content: &str,
         media: Vec<crate::bus::MediaItem>,
     ) -> Result<HandleResult, AgentError> {
-        // Determine dialog_id: if not provided, use current session or find active or create new
-        let unified_id = if let Some(did) = dialog_id {
-            UnifiedSessionId::new(channel, chat_id, did)
-        } else {
-            // Check if we have a current session tracked for this channel:chat_id
+        // Channel messages never carry dialog_id — routing is entirely via current_sessions
+        let unified_id = {
             let chat_scope = format!("{}:{}", channel, chat_id);
             let current_session_id = {
                 let inner = self.inner.lock().await;
@@ -1056,8 +1052,8 @@ impl SessionManager {
             if let Some(current_id) = current_session_id {
                 // Verify current session still exists in Storage
                 match self.storage.get_session(&current_id).await {
-                    Ok(meta) => {
-                        // Current session still valid
+                    Ok(_) => {
+                        // Current session still valid, extract dialog_id
                         let parts: Vec<&str> = current_id.split(':').collect();
                         if parts.len() == 3 {
                             UnifiedSessionId::new(channel, chat_id, parts[2])