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/agent-runner.ts

@@ -136,6 +136,7 @@ export async function runReplyAgent(params: {
     followupRun.run.config,
     replyToChannel,
     sessionCtx.AccountId,
+    sessionCtx.ChatType,
   );
   const applyReplyToMode = createReplyToModeFilterForChannel(replyToMode, replyToChannel);
   const cfg = followupRun.run.config;