Refactor the Channel trait to accept a SendMessage struct instead of
separate message and recipient string parameters. This enables passing
additional metadata like email subjects.
Changes:
- Add SendMessage struct with content, recipient, and optional subject
- Update Channel::send() signature to accept &SendMessage
- Update all 12 channel implementations
- Update call sites in channels/mod.rs and gateway/mod.rs
Subject field usage:
- Email: uses subject for email subject line
- DingTalk: uses subject as markdown message title
- All others: ignore subject (no native platform support)
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.
Spawns a repeating task that fires the Discord typing endpoint every
8 seconds while the LLM processes a response. Adds start_typing and
stop_typing to the Channel trait with default no-op impls so other
channels can opt in later.
