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/slack/accounts.ts

@@ -107,3 +107,13 @@ export function listEnabledSlackAccounts(cfg: ClawdbotConfig): ResolvedSlackAcco
     .map((accountId) => resolveSlackAccount({ cfg, accountId }))
     .filter((account) => account.enabled);
 }
+
+export function resolveSlackReplyToMode(
+  account: ResolvedSlackAccount,
+  chatType?: string | null,
+): "off" | "first" | "all" {
+  if (chatType === "direct" && account.dm?.replyToMode !== undefined) {
+    return account.dm.replyToMode;
+  }
+  return account.replyToMode ?? "off";
+}