fix(telegram): add human-like writing instructions for group chats

Port missing group chat style guidance from OpenClaw TS:
- "Write like a human" — prevents robotic/formal GPT responses
- "Avoid Markdown tables" — GPT tends to spam tables in groups
- "Use real line breaks sparingly"

These instructions exist in OpenClaw TS (groups.ts buildGroupIntro)
but were missing in GoClaw's group prompt.

cmd/gateway_consumer_normal.go

@@ -241,6 +241,7 @@ func processNormalMessage(
 			"- Messages may include a [Chat messages since your last reply] section with recent group history. Each history line shows \"sender [time]: message\".\n" +
 			"- The current message includes a [From: sender_name] tag identifying who @mentioned you.\n" +
 			"- Keep responses concise and focused; long replies are disruptive in groups.\n" +
+			"- Write like a human. Avoid Markdown tables. Use real line breaks sparingly.\n" +
 			"- Address the group naturally. If the history shows a multi-person conversation, consider the full context before answering."
 	}