feat(slack): add opt-in slack.strict_mention gate for channel threads

Adds a strict_mention config option that, when enabled, requires an
explicit @-mention on every message in channel threads. Disables the
'once mentioned, forever in the thread' and session-presence auto-triggers.

- New _slack_strict_mention() helper (config.extra + SLACK_STRICT_MENTION env)
- Bridged top-level slack.strict_mention yaml to SLACK_STRICT_MENTION env,
  matching require_mention/allow_bots bridging
- Unit tests for the helper + config bridge

gateway/config.py

@@ -611,6 +611,8 @@ def load_gateway_config() -> GatewayConfig:
             if isinstance(slack_cfg, dict):
                 if "require_mention" in slack_cfg and not os.getenv("SLACK_REQUIRE_MENTION"):
                     os.environ["SLACK_REQUIRE_MENTION"] = str(slack_cfg["require_mention"]).lower()
+                if "strict_mention" in slack_cfg and not os.getenv("SLACK_STRICT_MENTION"):
+                    os.environ["SLACK_STRICT_MENTION"] = str(slack_cfg["strict_mention"]).lower()
                 if "allow_bots" in slack_cfg and not os.getenv("SLACK_ALLOW_BOTS"):
                     os.environ["SLACK_ALLOW_BOTS"] = str(slack_cfg["allow_bots"]).lower()
                 frc = slack_cfg.get("free_response_channels")

gateway/platforms/slack.py

@@ -1133,6 +1133,8 @@ class SlackAdapter(BasePlatformAdapter):
                 pass  # Free-response channel — always process
             elif not self._slack_require_mention():
                 pass  # Mention requirement disabled globally for Slack
+            elif self._slack_strict_mention() and not is_mentioned:
+                return  # Strict mode: ignore until @-mentioned again
             elif not is_mentioned:
                 reply_to_bot_thread = (
                     is_thread_reply and event_thread_ts in self._bot_message_ts
@@ -1783,6 +1785,18 @@ class SlackAdapter(BasePlatformAdapter):
             return bool(configured)
         return os.getenv("SLACK_REQUIRE_MENTION", "true").lower() not in ("false", "0", "no", "off")
 
+    def _slack_strict_mention(self) -> bool:
+        """When true, channel threads require an explicit @-mention on every
+        message. Disables all auto-triggers (mentioned-thread memory,
+        bot-message follow-up, session-presence). Defaults to False.
+        """
+        configured = self.config.extra.get("strict_mention")
+        if configured is not None:
+            if isinstance(configured, str):
+                return configured.lower() in ("true", "1", "yes", "on")
+            return bool(configured)
+        return os.getenv("SLACK_STRICT_MENTION", "false").lower() in ("true", "1", "yes", "on")
+
     def _slack_free_response_channels(self) -> set:
         """Return channel IDs where no @mention is required."""
         raw = self.config.extra.get("free_response_channels")