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/config/types.slack.ts

@@ -17,6 +17,8 @@ export type SlackDmConfig = {
   groupEnabled?: boolean;
   /** Optional allowlist for group DM channels (ids or slugs). */
   groupChannels?: Array<string | number>;
+  /** Control reply threading for DMs (off|first|all). Overrides top-level replyToMode for DMs. */
+  replyToMode?: ReplyToMode;
 };
 
 export type SlackChannelConfig = {