fix(discord): document mention formatting guidance (#75173)

Summary:
- This PR adds Discord outbound mention-format guidance to the Discord plugin prompt hints, Discord channel docs, a focused channel test, and the changelog.
- Reproducibility: not applicable. as a runtime bug reproduction. There is a high-confidence inspection path:  ... lacks the guidance, while the existing formatter/tests and Discord reference establish the expected syntax.

ClawSweeper fixups:
- Included follow-up commit: fix(discord): document mention formatting guidance

Validation:
- ClawSweeper review passed for head bf2734a002.
- Required merge gates passed before the squash merge.

Prepared head SHA: bf2734a002
Review: https://github.com/openclaw/openclaw/pull/75173#issuecomment-4354537199

Co-authored-by: Peter Steinberger <steipete@gmail.com>
Co-authored-by: clawsweeper-repair <clawsweeper-repair@users.noreply.github.com>
Co-authored-by: Peter Steinberger <58493+steipete@users.noreply.github.com>

docs/channels/discord.md

@@ -579,6 +579,8 @@ Example:
     - configured mention patterns (`agents.list[].groupChat.mentionPatterns`, fallback `messages.groupChat.mentionPatterns`)
     - implicit reply-to-bot behavior in supported cases
 
+    When writing outbound Discord messages, use canonical mention syntax: `<@USER_ID>` for users, `<#CHANNEL_ID>` for channels, and `<@&ROLE_ID>` for roles. Do not use the legacy `<@!USER_ID>` nickname mention form.
+
     `requireMention` is configured per guild/channel (`channels.discord.guilds...`).
     `ignoreOtherMentions` optionally drops messages that mention another user/role but not the bot (excluding @everyone/@here).