feat(slack): add dm-specific replyToMode configuration (#1442)

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.

src/channels/plugins/types.core.ts

@@ -198,6 +198,7 @@ export type ChannelThreadingAdapter = {
   resolveReplyToMode?: (params: {
     cfg: ClawdbotConfig;
     accountId?: string | null;
+    chatType?: string | null;
   }) => "off" | "first" | "all";
   allowTagsWhenOff?: boolean;
   buildToolContext?: (params: {