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.

extensions/slack/src/channel.ts

@@ -19,6 +19,7 @@ import {
   readStringParam,
   resolveDefaultSlackAccountId,
   resolveSlackAccount,
+  resolveSlackReplyToMode,
   resolveSlackGroupRequireMention,
   buildSlackThreadingToolContext,
   setAccountEnabledInConfigSection,
@@ -162,8 +163,8 @@ export const slackPlugin: ChannelPlugin<ResolvedSlackAccount> = {
     resolveRequireMention: resolveSlackGroupRequireMention,
   },
   threading: {
-    resolveReplyToMode: ({ cfg, accountId }) =>
-      resolveSlackAccount({ cfg, accountId }).replyToMode ?? "off",
+    resolveReplyToMode: ({ cfg, accountId, chatType }) =>
+      resolveSlackReplyToMode(resolveSlackAccount({ cfg, accountId }), chatType),
     allowTagsWhenOff: true,
     buildToolContext: (params) => buildSlackThreadingToolContext(params),
   },