fix: harden Mattermost plugin gating (#1428) (thanks @damoahdominic)

docs/cli/message.md

@@ -8,7 +8,7 @@ read_when:
 # `clawdbot message`
 
 Single outbound command for sending messages and channel actions
-(Discord/Slack/Mattermost/Telegram/WhatsApp/Signal/iMessage/MS Teams).
+(Discord/Slack/Mattermost (plugin)/Telegram/WhatsApp/Signal/iMessage/MS Teams).
 
 ## Usage
 
@@ -19,14 +19,14 @@ clawdbot message <subcommand> [flags]
 Channel selection:
 - `--channel` required if more than one channel is configured.
 - If exactly one channel is configured, it becomes the default.
-- Values: `whatsapp|telegram|discord|slack|mattermost|signal|imessage|msteams`
+- Values: `whatsapp|telegram|discord|slack|mattermost|signal|imessage|msteams` (Mattermost requires plugin)
 
 Target formats (`--target`):
 - WhatsApp: E.164 or group JID
 - Telegram: chat id or `@username`
 - Discord: `channel:<id>` or `user:<id>` (or `<@id>` mention; raw numeric ids are treated as channels)
 - Slack: `channel:<id>` or `user:<id>` (raw channel id is accepted)
-- Mattermost: `channel:<id>`, `user:<id>`, or `@username` (bare ids are treated as channels)
+- Mattermost (plugin): `channel:<id>`, `user:<id>`, or `@username` (bare ids are treated as channels)
 - Signal: `+E.164`, `group:<id>`, `signal:+E.164`, `signal:group:<id>`, or `username:<name>`/`u:<name>`
 - iMessage: handle, `chat_id:<id>`, `chat_guid:<guid>`, or `chat_identifier:<id>`
 - MS Teams: conversation id (`19:...@thread.tacv2`) or `conversation:<id>` or `user:<aad-object-id>`
@@ -50,7 +50,7 @@ Name lookup:
 ### Core
 
 - `send`
-  - Channels: WhatsApp/Telegram/Discord/Slack/Mattermost/Signal/iMessage/MS Teams
+  - Channels: WhatsApp/Telegram/Discord/Slack/Mattermost (plugin)/Signal/iMessage/MS Teams
   - Required: `--target`, plus `--message` or `--media`
   - Optional: `--media`, `--reply-to`, `--thread-id`, `--gif-playback`
   - Telegram only: `--buttons` (requires `channels.telegram.capabilities.inlineButtons` to allow it)