The Slack channel plugin's read action handler was not forwarding the threadId parameter to handleSlackAction, causing message action=read channel=slack threadId=X to always return channel history instead of thread replies.
Adds support for separate replyToMode settings for DMs vs channels:
- Add channels.slack.dm.replyToMode for DM-specific threading
- Keep channels.slack.replyToMode as default for channels
- Add resolveSlackReplyToMode helper to centralize logic
- Pass chatType through threading resolution chain
Usage:
```json5
{
channels: {
slack: {
replyToMode: "off", // channels
dm: {
replyToMode: "all" // DMs always thread
}
}
}
}
```
When dm.replyToMode is set, DMs use that mode; channels use the
top-level replyToMode. Backward compatible when not configured.
