fix(messages): keep group replies tool-only by default

Rewrites the always-on reply handling so group/channel rooms default to message-tool-visible output, while `messages.groupChat.visibleReplies: \"automatic\"` preserves legacy auto-posting.\n\nThanks @scoootscooob.

docs/channels/groups.md

@@ -16,6 +16,7 @@ Default behavior:
 
 - Groups are restricted (`groupPolicy: "allowlist"`).
 - Replies require a mention unless you explicitly disable mention gating.
+- Normal final replies in groups/channels are private by default. Visible room output uses the `message` tool.
 
 Translation: allowlisted senders can trigger OpenClaw by mentioning it.
 
@@ -36,6 +37,25 @@ requireMention? yes -> mentioned? no -> store for context only
 otherwise -> reply
 ```
 
+## Visible replies
+
+For group/channel rooms, OpenClaw defaults to `messages.groupChat.visibleReplies: "message_tool"`.
+That means the agent still processes the turn and can update memory/session state, but its normal final answer is not automatically posted back into the room. To speak visibly, the agent uses `message(action=send)`.
+
+This replaces the old pattern of forcing the model to answer `NO_REPLY` for most lurk-mode turns. In tool-only mode, doing nothing visible simply means not calling the message tool.
+
+To restore legacy automatic final replies for group/channel rooms:
+
+```json5
+{
+  messages: {
+    groupChat: {
+      visibleReplies: "automatic",
+    },
+  },
+}
+```
+
 ## Context visibility and allowlists
 
 Two different controls are involved in group safety: