Users commonly place `require_mention: true` at the top level of
config.yaml alongside `group_sessions_per_user`, expecting it to gate
Telegram group messages. The key was silently ignored because the
config loader only checked `yaml_cfg["telegram"]["require_mention"]`.
When `require_mention` is found at the top level and no telegram-specific
value is set, the fix now:
- adds it to platforms_data["telegram"]["extra"] so _telegram_require_mention()
picks it up via the primary config.extra path
- sets TELEGRAM_REQUIRE_MENTION env var for the secondary fallback path
A telegram-specific value (telegram.require_mention) still takes
precedence over the top-level shorthand.
Also corrects telegram.md: bare /cmd without @botname is rejected when
require_mention is enabled; only /cmd@botname (bot-menu form) passes.
Fixes#3979
6fda92aa7f