Channel mention-gating was hardcoded on. Add a configurable require_mention
(default True, preserving current behavior). When False, the agent responds to
every message in a watched channel, not only when @mentioned; DMs always
dispatch. Read from config.yaml gateway.platforms.buzz.extra.require_mention
with BUZZ_REQUIRE_MENTION env override, bridged via apply_yaml_config_fn like
the other settings. A leading mention is still stripped when present.
Refs #68871
ffb38f0c03