Implement SSH Agent forwarding support

- Add auth_agent_socket field to Channel struct
- Add handle_auth_agent_request() for auth-agent-req@openssh.com
- Check SSH_AUTH_SOCK environment variable for agent socket
- Respond with SSH_MSG_CHANNEL_SUCCESS if agent available
- Foundation for SSH agent forwarding through jump hosts

All 229 tests pass.

markbase-core/src/ssh_server/channel.rs

@@ -183,6 +183,7 @@ impl ChannelManager {
             scp_output_file: None,           // Phase 17: SCP file receive
             direct_tcpip: None,
             forwarded_tcpip: None,
+            auth_agent_socket: None,
         };
 
         self.channels.insert(server_channel, channel);
@@ -263,6 +264,7 @@ impl ChannelManager {
             scp_output_file: None,           // Phase 17: SCP file receive
             direct_tcpip: Some(direct_tcpip),
             forwarded_tcpip: None,
+            auth_agent_socket: None,
         };
 
         self.channels.insert(server_channel, channel);
@@ -340,6 +342,7 @@ impl ChannelManager {
             scp_output_file: None,           // Phase 17: SCP file receive
             direct_tcpip: None,
             forwarded_tcpip: Some(forwarded_tcpip),
+            auth_agent_socket: None,
         };
 
         self.channels.insert(server_channel, channel);
@@ -409,6 +412,8 @@ impl ChannelManager {
             self.handle_env_request(&mut cursor, recipient_channel, want_reply) // 移除?操作符
         } else if request_type == "pty-req" {
             self.handle_pty_request(&mut cursor, recipient_channel, want_reply) // 移除?操作符
+        } else if request_type == "auth-agent-req@openssh.com" {
+            self.handle_auth_agent_request(recipient_channel, want_reply)
         } else {
             warn!("Unsupported channel request: {}", request_type);
             if want_reply {
@@ -702,6 +707,41 @@ impl ChannelManager {
         }
     }
 
+    /// SSH Agent forwarding（参考OpenSSH auth-agent.c）
+    /// 支持 "auth-agent-req@openssh.com" channel request
+    fn handle_auth_agent_request(
+        &mut self,
+        channel: u32,
+        want_reply: bool,
+    ) -> Result<Option<SshPacket>> {
+        info!("Handling auth-agent request for channel {}", channel);
+
+        // 检查SSH_AUTH_SOCK环境变量
+        let auth_sock = std::env::var("SSH_AUTH_SOCK").ok();
+        
+        if let Some(sock_path) = auth_sock {
+            info!("SSH Agent forwarding enabled: {}", sock_path);
+            
+            // 标记channel支持agent forwarding
+            if let Some(ch) = self.channels.get_mut(&channel) {
+                ch.auth_agent_socket = Some(sock_path);
+            }
+
+            if want_reply {
+                Ok(Some(self.build_channel_success(channel)?))
+            } else {
+                Ok(None)
+            }
+        } else {
+            warn!("SSH_AUTH_SOCK not set, agent forwarding disabled");
+            if want_reply {
+                Ok(Some(self.build_channel_failure(channel)?))
+            } else {
+                Ok(None)
+            }
+        }
+    }
+
     /// 处理SSH_MSG_CHANNEL_DATA（参考OpenSSH channel.c: channel_input_data())
     pub fn handle_channel_data(&mut self, packet: &SshPacket) -> Result<Option<SshPacket>> {
         info!("Processing SSH_MSG_CHANNEL_DATA");
@@ -2101,6 +2141,8 @@ struct Channel {
     // Phase 13.3: 端口转发相关字段
     direct_tcpip: Option<DirectTcpipChannel>, // direct-tcpip channel（Remote forwarding）
     forwarded_tcpip: Option<ForwardedTcpipChannel>, // forwarded-tcpip channel（Local forwarding）
+    // SSH Agent forwarding
+    auth_agent_socket: Option<String>, // SSH agent socket path (SSH_AUTH_SOCK)
 }
 
 /// SSH Channel状态（参考OpenSSH channel.c）