feat(session): 添加 /? 帮助命令，错误命令显示友好提示

- 新增 /? 和 /help 命令显示所有可用斜杠命令
- 未知命令不再报错，而是提示"未知命令：/xxx。输入 /? 获取帮助。"
- execute_slash_command 错误现在返回友好提示而非传播为服务器错误

src/channels/slash_command.rs

@@ -28,6 +28,10 @@ mod tests {
         assert_eq!(parse_slash_command("/reset"), Some(("reset", "")));
         assert_eq!(parse_slash_command("/reset arg"), Some(("reset", "arg")));
         assert_eq!(parse_slash_command("/new hello world"), Some(("new", "hello world")));
+        assert_eq!(parse_slash_command("/??"), Some(("??", "")));
+        assert_eq!(parse_slash_command("/? arg"), Some(("?", "arg")));
+        assert_eq!(parse_slash_command("/?"), Some(("?", "")));
+        assert_eq!(parse_slash_command("/help"), Some(("help", "")));
         assert_eq!(parse_slash_command("hello"), None);
         assert_eq!(parse_slash_command("/"), Some(("", "")));
     }

src/session/session.rs

@@ -637,6 +637,11 @@ pub static SLASH_COMMANDS: &[SlashCommand] = &[
         description: "保存当前对话为 markdown 文档",
         aliases: &["/dump"],
     },
+    SlashCommand {
+        name: "?",
+        description: "显示帮助",
+        aliases: &["/?", "/help"],
+    },
 ];
 
 impl SessionManager {
@@ -820,7 +825,13 @@ impl SessionManager {
                     Ok((None, "No active session.".to_string()))
                 }
             }
-            _ => Err(AgentError::Other(format!("Command not implemented: {}", cmd.name))),
+            "?" | "help" => {
+                let lines: Vec<String> = SLASH_COMMANDS.iter().map(|c| {
+                    format!("  {}  - {}", c.aliases.join(", "), c.description)
+                }).collect();
+                Ok((None, format!("可用命令：\n{}", lines.join("\n"))))
+            }
+            _ => Err(AgentError::Other(format!("未知命令：/{}。输入 /? 获取帮助。", cmd.name))),
         }
     }
 
@@ -1092,15 +1103,22 @@ impl SessionManager {
 
         // Check for slash command
         if let Some((cmd_name, cmd_args)) = parse_slash_command(content) {
-            let (new_session_id, response) = self.execute_slash_command(
+            let result = self.execute_slash_command(
                 cmd_name,
                 if cmd_args.is_empty() { None } else { Some(cmd_args) },
                 channel,
                 chat_id,
                 Some(&unified_id),
-            ).await?;
+            ).await;
 
-            return Ok(HandleResult::CommandOutput(response));
+            match result {
+                Ok((_new_session_id, response)) => {
+                    return Ok(HandleResult::CommandOutput(response));
+                }
+                Err(e) => {
+                    return Ok(HandleResult::CommandOutput(e.to_string()));
+                }
+            }
         }
 
         // Normal message handling through LLM