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/threading-tool-context.ts

@@ -3,7 +3,7 @@ import type {
   ChannelThreadingToolContext,
 } from "../channels/plugins/types.js";
 import type { ClawdbotConfig } from "../config/config.js";
-import { resolveSlackAccount } from "./accounts.js";
+import { resolveSlackAccount, resolveSlackReplyToMode } from "./accounts.js";
 
 export function buildSlackThreadingToolContext(params: {
   cfg: ClawdbotConfig;
@@ -11,11 +11,11 @@ export function buildSlackThreadingToolContext(params: {
   context: ChannelThreadingContext;
   hasRepliedRef?: { value: boolean };
 }): ChannelThreadingToolContext {
-  const configuredReplyToMode =
-    resolveSlackAccount({
-      cfg: params.cfg,
-      accountId: params.accountId,
-    }).replyToMode ?? "off";
+  const account = resolveSlackAccount({
+    cfg: params.cfg,
+    accountId: params.accountId,
+  });
+  const configuredReplyToMode = resolveSlackReplyToMode(account, params.context.ChatType);
   const effectiveReplyToMode = params.context.ThreadLabel ? "all" : configuredReplyToMode;
   const threadId = params.context.MessageThreadId ?? params.context.ReplyToId;
   return {