fix(mattermost): "@bot /new" in channels is answered as chat instead …

fix(mattermost): "@bot /new" in channels is answered as chat instead of running the command (#137902)

* fix(mattermost): "@bot /new" in channels is answered as chat instead of running the command

Command detection and CommandBody used the raw post text, which still
carried the leading bot mention; the core detector only recognizes a
leading slash, so "@bot /new" reached the model as prose. Strip the
mention first, as Slack does, for detection, CommandBody, and the
inbound debounce gate.

* docs(mattermost): qualify mention-prefixed text commands with commands.text

Mattermost registers native commands, so `commands.text: false` disables
`/...` parsing in chat posts there. State the prerequisite instead of
promising that `@ /new` always runs.

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Co-authored-by: goffern