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

gateway/config.py

@@ -846,11 +846,25 @@ def load_gateway_config() -> GatewayConfig:
                         if yaml_key in allow_mentions_cfg and not os.getenv(env_key):
                             os.environ[env_key] = str(allow_mentions_cfg[yaml_key]).lower()
 
+            # Bridge top-level require_mention to Telegram when the telegram: section
+            # does not already provide one.  Users often write "require_mention: true"
+            # at the top level alongside group_sessions_per_user, expecting it to work
+            # the same way (#3979).
+            _tl_require_mention = yaml_cfg.get("require_mention")
+            if _tl_require_mention is not None:
+                _tg_section = yaml_cfg.get("telegram") or {}
+                if "require_mention" not in _tg_section:
+                    _tg_plat = platforms_data.setdefault(Platform.TELEGRAM.value, {})
+                    _tg_extra = _tg_plat.setdefault("extra", {})
+                    _tg_extra.setdefault("require_mention", _tl_require_mention)
+
             # Telegram settings → env vars (env vars take precedence)
             telegram_cfg = yaml_cfg.get("telegram", {})
             if isinstance(telegram_cfg, dict):
-                if "require_mention" in telegram_cfg and not os.getenv("TELEGRAM_REQUIRE_MENTION"):
-                    os.environ["TELEGRAM_REQUIRE_MENTION"] = str(telegram_cfg["require_mention"]).lower()
+                # Prefer telegram.require_mention; fall back to the top-level shorthand.
+                _effective_rm = telegram_cfg.get("require_mention", yaml_cfg.get("require_mention"))
+                if _effective_rm is not None and not os.getenv("TELEGRAM_REQUIRE_MENTION"):
+                    os.environ["TELEGRAM_REQUIRE_MENTION"] = str(_effective_rm).lower()
                 if "mention_patterns" in telegram_cfg and not os.getenv("TELEGRAM_MENTION_PATTERNS"):
                     os.environ["TELEGRAM_MENTION_PATTERNS"] = json.dumps(telegram_cfg["mention_patterns"])
                 frc = telegram_cfg.get("free_response_chats")