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.

docs/channels/slack.md

@@ -304,7 +304,8 @@ Slack uses Socket Mode only (no HTTP webhook server). Provide both tokens:
       "policy": "pairing",
       "allowFrom": ["U123", "U456", "*"],
       "groupEnabled": false,
-      "groupChannels": ["G123"]
+      "groupChannels": ["G123"],
+      "replyToMode": "all"
     },
     "channels": {
       "C123": { "allow": true, "requireMention": true },
@@ -361,6 +362,24 @@ By default, Clawdbot replies in the main channel. Use `channels.slack.replyToMod
 
 The mode applies to both auto-replies and agent tool calls (`slack sendMessage`).
 
+### DM-specific threading
+You can configure different threading behavior for DMs vs channels by setting `channels.slack.dm.replyToMode`:
+
+```json5
+{
+  channels: {
+    slack: {
+      replyToMode: "off",        // default for channels
+      dm: {
+        replyToMode: "all"       // DMs always thread
+      }
+    }
+  }
+}
+```
+
+When `dm.replyToMode` is set, DMs use that mode; channels use the top-level `replyToMode`. If `dm.replyToMode` is not set, both DMs and channels use the top-level setting.
+
 ### Manual threading tags
 For fine-grained control, use these tags in agent responses:
 - `[[reply_to_current]]` — reply to the triggering message (start/continue thread).