Implement SSH Phase 6: Channel protocol with command execution

Phase 6 completed:
- SSH_MSG_CHANNEL_OPEN handling
- SSH_MSG_CHANNEL_OPEN_CONFIRMATION/FAILURE responses
- SSH_MSG_CHANNEL_REQUEST handling (exec, env, shell, subsystem)
- SSH_MSG_CHANNEL_DATA transmission (command output)
- SSH_MSG_CHANNEL_EOF/CLOSE handling
- Command execution via shell (sh -c)
- Encrypted packet handling in service loop

Test results:
- SSH connection successful with channel creation
- Command execution working: 'echo', 'whoami', 'pwd', 'ls'
- Output correctly transmitted via CHANNEL_DATA
- EOF and CLOSE properly sent after execution
- Multiple commands working correctly

Files modified:
- channel.rs: Channel management, command execution, output buffering
- server.rs: Encrypted service loop, channel output handling

Progress: SSH implementation 95% complete (Phase 1-6)

markbase-core/src/ssh_server/channel.rs

@@ -72,6 +72,7 @@ impl ChannelManager {
             window_size: initial_window_size,
             maximum_packet_size,
             state: ChannelState::Open,
+            output_buffer: None,  // Phase 6: 初始化为空
         };
         
         self.channels.insert(server_channel, channel);
@@ -136,7 +137,14 @@ impl ChannelManager {
         
         info!("Exec command: {}", command);
         
-        // 简化实现：返回成功（实际应执行命令）
+        // 执行命令（Phase 6实现基础命令执行）
+        let output = self.execute_command(&command)?;
+        
+        // 存储输出，等待后续发送CHANNEL_DATA
+        if let Some(ch) = self.channels.get_mut(&channel) {
+            ch.output_buffer = Some(output);
+        }
+        
         if want_reply {
             Ok(Some(self.build_channel_success(channel)?))
         } else {
@@ -144,6 +152,26 @@ impl ChannelManager {
         }
     }
     
+    /// 执行命令并捕获输出（Phase 6基础实现）
+    fn execute_command(&self, command: &str) -> Result<Vec<u8>> {
+        use std::process::{Command, Stdio};
+        
+        info!("Executing command: {}", command);
+        
+        // 使用shell执行命令（参考OpenSSH session.c）
+        let output = Command::new("sh")
+            .arg("-c")
+            .arg(command)
+            .output()?;
+        
+        // 返回stdout + stderr
+        let mut result = output.stdout;
+        result.extend_from_slice(&output.stderr);
+        
+        info!("Command output: {} bytes", result.len());
+        Ok(result)
+    }
+    
     /// 处理subsystem请求（参考OpenSSH channel.c: channel_request_subsystem())
     fn handle_subsystem_request(&mut self, cursor: &mut std::io::Cursor<&[u8]>, channel: u32, want_reply: bool) -> Result<Option<SshPacket>> {
         info!("Handling subsystem request for channel {}", channel);
@@ -362,7 +390,7 @@ impl ChannelManager {
     }
     
     /// 构建SSH_MSG_CHANNEL_CLOSE（参考OpenSSH channel.c）
-    fn build_channel_close(&self, channel: u32) -> Result<SshPacket> {
+    pub fn build_channel_close(&self, channel: u32) -> Result<SshPacket> {
         let mut payload = Vec::new();
         
         payload.write_u8(PacketType::SSH_MSG_CHANNEL_CLOSE as u8)?;
@@ -370,6 +398,52 @@ impl ChannelManager {
         
         Ok(SshPacket::new(payload))
     }
+    
+    /// 构建SSH_MSG_CHANNEL_DATA（Phase 6新增）
+    pub fn build_channel_data(&self, channel: u32, data: &[u8]) -> Result<SshPacket> {
+        let mut payload = Vec::new();
+        
+        payload.write_u8(PacketType::SSH_MSG_CHANNEL_DATA as u8)?;
+        payload.write_u32::<BigEndian>(channel)?;
+        payload.write_u32::<BigEndian>(data.len() as u32)?;
+        payload.write_all(data)?;
+        
+        Ok(SshPacket::new(payload))
+    }
+    
+    /// 构建SSH_MSG_CHANNEL_EOF（Phase 6新增）
+    pub fn build_channel_eof(&self, channel: u32) -> Result<SshPacket> {
+        let mut payload = Vec::new();
+        
+        payload.write_u8(PacketType::SSH_MSG_CHANNEL_EOF as u8)?;
+        payload.write_u32::<BigEndian>(channel)?;
+        
+        Ok(SshPacket::new(payload))
+    }
+    
+    /// 获取有输出待发送的channel ID（Phase 6新增）
+    pub fn get_channel_with_output(&self) -> Option<u32> {
+        for (&id, channel) in &self.channels {
+            if channel.output_buffer.is_some() {
+                return Some(id);
+            }
+        }
+        None
+    }
+    
+    /// 获取channel输出（Phase 6新增）
+    pub fn get_channel_output(&mut self, channel_id: u32) -> Option<Vec<u8>> {
+        if let Some(channel) = self.channels.get_mut(&channel_id) {
+            channel.output_buffer.take()
+        } else {
+            None
+        }
+    }
+    
+    /// 移除channel（Phase 6新增）
+    pub fn remove_channel(&mut self, channel_id: u32) {
+        self.channels.remove(&channel_id);
+    }
 }
 
 /// SSH Channel结构（参考OpenSSH channel.c: struct channel）
@@ -380,6 +454,7 @@ struct Channel {
     window_size: u32,
     maximum_packet_size: u32,
     state: ChannelState,
+    output_buffer: Option<Vec<u8>>,  // Phase 6: 命令输出缓冲
 }
 
 /// SSH Channel状态（参考OpenSSH channel.c）