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/telegram.rs

@@ -598,6 +598,7 @@ Allowlist Telegram @username or numeric user ID, then run `zeroclaw onboard --ch
                     let msg = ChannelMessage {
                         id: format!("telegram_{chat_id}_{message_id}"),
                         sender: username.to_string(),
+                        reply_to: chat_id.clone(),
                         content: text.to_string(),
                         channel: "telegram".to_string(),
                         timestamp: std::time::SystemTime::now()