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/auto-reply/reply/queue/types.ts

@@ -39,6 +39,8 @@ export type FollowupRun = {
   originatingAccountId?: string;
   /** Thread id for reply routing (Telegram topic id or Matrix thread event id). */
   originatingThreadId?: string | number;
+  /** Chat type for context-aware threading (e.g., DM vs channel). */
+  originatingChatType?: string;
   run: {
     agentId: string;
     agentDir: string;