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/channels/dock.ts

@@ -2,7 +2,7 @@ import type { ClawdbotConfig } from "../config/config.js";
 import { resolveDiscordAccount } from "../discord/accounts.js";
 import { resolveIMessageAccount } from "../imessage/accounts.js";
 import { resolveSignalAccount } from "../signal/accounts.js";
-import { resolveSlackAccount } from "../slack/accounts.js";
+import { resolveSlackAccount, resolveSlackReplyToMode } from "../slack/accounts.js";
 import { buildSlackThreadingToolContext } from "../slack/threading-tool-context.js";
 import { resolveTelegramAccount } from "../telegram/accounts.js";
 import { normalizeE164 } from "../utils.js";
@@ -224,8 +224,8 @@ const DOCKS: Record<ChatChannelId, ChannelDock> = {
       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),
     },

src/channels/plugins/types.core.ts

@@ -198,6 +198,7 @@ export type ChannelThreadingAdapter = {
   resolveReplyToMode?: (params: {
     cfg: ClawdbotConfig;
     accountId?: string | null;
+    chatType?: string | null;
   }) => "off" | "first" | "all";
   allowTagsWhenOff?: boolean;
   buildToolContext?: (params: {