feat(signal): add require_mention filter for group chats

Add a configurable mention filter to the Signal adapter so the bot
only responds in groups when it is explicitly @mentioned.

Changes:
- gateway/platforms/signal.py: read require_mention from adapter
  extra config or SIGNAL_REQUIRE_MENTION env var; skip group messages
  that don't mention the bot account (checked in rendered text and
  raw mention metadata)
- gateway/config.py: map signal.require_mention YAML key to the
  SIGNAL_REQUIRE_MENTION env var (env var takes precedence)

Config example:
  signal:
    require_mention: true

Or via env var:
  SIGNAL_REQUIRE_MENTION=true

gateway/config.py

@@ -1118,6 +1118,12 @@ def load_gateway_config() -> GatewayConfig:
                         gaf = ",".join(str(v) for v in gaf)
                     os.environ["WHATSAPP_GROUP_ALLOWED_USERS"] = str(gaf)
 
+            # Signal settings → env vars (env vars take precedence)
+            signal_cfg = yaml_cfg.get("signal", {})
+            if isinstance(signal_cfg, dict):
+                if "require_mention" in signal_cfg and not os.getenv("SIGNAL_REQUIRE_MENTION"):
+                    os.environ["SIGNAL_REQUIRE_MENTION"] = str(signal_cfg["require_mention"]).lower()
+
             # DingTalk settings → env vars (env vars take precedence)
             dingtalk_cfg = yaml_cfg.get("dingtalk", {})
             if isinstance(dingtalk_cfg, dict):

gateway/platforms/signal.py

@@ -192,6 +192,14 @@ class SignalAdapter(BasePlatformAdapter):
         group_allowed_str = os.getenv("SIGNAL_GROUP_ALLOWED_USERS", "")
         self.group_allow_from = set(_parse_comma_list(group_allowed_str))
 
+        # Mention filter — only respond in groups when the bot account is @mentioned.
+        # Read from config extra first, then SIGNAL_REQUIRE_MENTION env var.
+        _rm_cfg = extra.get("require_mention")
+        if _rm_cfg is not None:
+            self.require_mention = bool(_rm_cfg)
+        else:
+            self.require_mention = os.getenv("SIGNAL_REQUIRE_MENTION", "false").lower() in ("true", "1", "yes", "on")
+
         # DM allowlist — mirrors SIGNAL_ALLOWED_USERS checked by run.py.
         # Stored here so the reaction hooks can skip unauthorized senders
         # (reactions fire before run.py's auth gate, so without this check
@@ -528,6 +536,23 @@ class SignalAdapter(BasePlatformAdapter):
         if text and mentions:
             text = _render_mentions(text, mentions)
 
+        # Mention filter: in groups, only process messages that @mention the bot account
+        if is_group and self.require_mention:
+            account_norm = self._account_normalized
+            # Check rendered mention tags OR raw mention metadata
+            mentioned_in_text = account_norm and (
+                f"@{account_norm}" in (text or "")
+            )
+            mentioned_in_metadata = any(
+                m.get("number") == account_norm or m.get("uuid") == account_norm
+                for m in (data_message.get("mentions") or [])
+            )
+            if not mentioned_in_text and not mentioned_in_metadata:
+                logger.debug(
+                    "Signal: ignoring group message (require_mention=true, bot not mentioned)"
+                )
+                return
+
         # Extract quote (reply-to) context from Signal dataMessage
         quote_data = data_message.get("quote") or {}
         reply_to_id = str(quote_data.get("id")) if quote_data.get("id") else None