fix(channels): add reply_to field to ChannelMessage for correct reply routing

ChannelMessage.sender was used both for display (username) and as the
reply target in Channel::send(). For Telegram, sender is the username
(e.g. "unknown") while send() requires the numeric chat_id, causing
"Bad Request: chat not found" errors.

Add a dedicated reply_to field to ChannelMessage that stores the
channel-specific reply address (Telegram chat_id, Discord channel_id,
Slack channel, etc.). Update all channel implementations and dispatch
code to use reply_to for send/start_typing/stop_typing calls.

This also fixes the same latent bug in Discord and Slack channels where
sender (user ID) was incorrectly passed as the reply target.

src/channels/dingtalk.rs

@@ -229,6 +229,7 @@ impl Channel for DingTalkChannel {
                     let channel_msg = ChannelMessage {
                         id: Uuid::new_v4().to_string(),
                         sender: sender_id.to_string(),
+                        reply_to: sender_id.to_string(),
                         content: content.to_string(),
                         channel: "dingtalk".to_string(),
                         timestamp: std::time::SystemTime::now()