Docs: add Discord forum thread docs

docs/channels/discord.md

@@ -258,6 +258,29 @@ Now create some channels on your Discord server and start chatting. Your agent c
 - Group DMs are ignored by default (`channels.discord.dm.groupEnabled=false`).
 - Native slash commands run in isolated command sessions (`agent:<agentId>:discord:slash:<userId>`), while still carrying `CommandTargetSessionKey` to the routed conversation session.
 
+## Forum channels
+
+Discord forum and media channels only accept thread posts. OpenClaw supports two ways to create them:
+
+- Send a message to the forum parent (`channel:<forumId>`) to auto-create a thread. The thread title uses the first non-empty line of your message.
+- Use `openclaw message thread create` to create a thread directly. Do not pass `--message-id` for forum channels.
+
+Example: send to forum parent to create a thread
+
+```bash
+openclaw message send --channel discord --target channel:<forumId> \
+  --message "Topic title\nBody of the post"
+```
+
+Example: create a forum thread explicitly
+
+```bash
+openclaw message thread create --channel discord --target channel:<forumId> \
+  --thread-name "Topic title" --message "Body of the post"
+```
+
+Forum parents do not accept Discord components. If you need components, send to the thread itself (`channel:<threadId>`).
+
 ## Interactive components
 
 OpenClaw supports Discord components v2 containers for agent messages. Use the message tool with a `components` payload. Interaction results are routed back to the agent as normal inbound messages and follow the existing Discord `replyToMode` settings.