feat(mattermost): configurable mention behavior — respond without @mention

[teknium1]

Summary

Adds configurable mention gating for Mattermost channels, matching Discord's existing pattern. Requested by community member neeldhara.

New Environment Variables

Variable	Default	Description
MATTERMOST_REQUIRE_MENTION	true	Set to false to respond to all channel messages without @mention
MATTERMOST_FREE_RESPONSE_CHANNELS	(none)	Comma-separated channel IDs where bot responds without @mention

DMs always work regardless of these settings.

Usage

# Respond to all messages in all channels
MATTERMOST_REQUIRE_MENTION=false

# Or: respond without mention only in specific channels
MATTERMOST_FREE_RESPONSE_CHANNELS=channel_id_1,channel_id_2

Also fixes

@mention is now stripped from the message text before the agent sees it (previously the raw @botname was included in the prompt).

Files Changed

File	Change
gateway/platforms/mattermost.py	Mention gating logic + mention stripping
hermes_cli/config.py	Env var metadata for setup/doctor
tests/gateway/test_mattermost.py	7 new tests + 1 updated test
website/docs/user-guide/messaging/mattermost.md	Mention Behavior section + env var examples
website/docs/reference/environment-variables.md	New vars in reference table

Tests

46 Mattermost tests pass (7 new).

[neeldhara]

This is amazing, thank you so much!

I am not sure if this is the right place to suggest this, but a minor enhancement would be the following: to be able to specify a list of channels where an @-mention is required. In my use case, most channels are with one agent and I would prefer no mentions required, 'but a couple of them are agent groups where mentions make sense.