fix(gateway): bridge top-level require_mention to Telegram config

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

website/docs/user-guide/messaging/telegram.md

@@ -293,9 +293,9 @@ Hermes Agent works in Telegram group chats with a few considerations:
 - `TELEGRAM_ALLOWED_USERS` still applies — only authorized users can trigger the bot, even in groups
 - You can keep the bot from responding to ordinary group chatter with `telegram.require_mention: true`
 - With `telegram.require_mention: true`, group messages are accepted when they are:
-  - slash commands
   - replies to one of the bot's messages
   - `@botusername` mentions
+  - `/command@botusername` (Telegram's bot-menu command form that includes the bot name)
   - matches for one of your configured regex wake words in `telegram.mention_patterns`
 - Use `telegram.ignored_threads` to keep Hermes silent in specific Telegram forum topics, even when the group would otherwise allow free responses or mention-triggered replies
 - If `telegram.require_mention` is left unset or false, Hermes keeps the previous open-group behavior and responds to normal group messages it can see