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/plugin-sdk/index.ts

@@ -230,6 +230,7 @@ export {
   listSlackAccountIds,
   resolveDefaultSlackAccountId,
   resolveSlackAccount,
+  resolveSlackReplyToMode,
   type ResolvedSlackAccount,
 } from "../slack/accounts.js";
 export { slackOnboardingAdapter } from "../channels/plugins/onboarding/slack.js";