hermes - 💡(How to fix) Fix Harden Slack and Mattermost mass mention handling

Summary

Slack and Mattermost gateway adapters should default to non-executable broadcast mention delivery for bot-authored responses. If model output contains Slack broadcast mentions such as <!everyone>, <!channel>, or <!here>, Slack can receive them as mrkdwn entities. Mattermost posts sent without props.disable_mentions=true can allow @all / @channel to be parsed by the server.

Under the current Hermes threat model this is best treated as defense-in-depth output hardening rather than a standalone prompt-injection boundary bypass, but default-safe platform delivery prevents accidental or prompt-induced notification storms in shared workspaces.

Impact

• Bot responses can trigger workspace/channel-wide notifications when model output includes broadcast mention syntax.
• This can cause notification spam and operational disruption in large shared Slack or Mattermost channels.

Suggested fix

• Slack: escape broadcast mentions before preserving Slack entities for mrkdwn formatting.
• Mattermost: include props: {"disable_mentions": true} in bot-authored post and patch payloads.

Validation

A regression PR is being opened with targeted tests for:

• Slack formatting of <!everyone>, <!channel>, and <!here> as literal text.
• Mattermost send payloads setting props.disable_mentions=true.