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")

tests/gateway/test_slack_mention.py

@@ -55,10 +55,12 @@ CHANNEL_ID = "C0AQWDLHY9M"
 OTHER_CHANNEL_ID = "C9999999999"
 
 
-def _make_adapter(require_mention=None, free_response_channels=None):
+def _make_adapter(require_mention=None, strict_mention=None, free_response_channels=None):
     extra = {}
     if require_mention is not None:
         extra["require_mention"] = require_mention
+    if strict_mention is not None:
+        extra["strict_mention"] = strict_mention
     if free_response_channels is not None:
         extra["free_response_channels"] = free_response_channels
 
@@ -134,6 +136,48 @@ def test_require_mention_env_var_default_true(monkeypatch):
     assert adapter._slack_require_mention() is True
 
 
+# ---------------------------------------------------------------------------
+# Tests: _slack_strict_mention
+# ---------------------------------------------------------------------------
+
+def test_strict_mention_defaults_to_false(monkeypatch):
+    monkeypatch.delenv("SLACK_STRICT_MENTION", raising=False)
+    adapter = _make_adapter()
+    assert adapter._slack_strict_mention() is False
+
+
+def test_strict_mention_true():
+    adapter = _make_adapter(strict_mention=True)
+    assert adapter._slack_strict_mention() is True
+
+
+def test_strict_mention_false():
+    adapter = _make_adapter(strict_mention=False)
+    assert adapter._slack_strict_mention() is False
+
+
+def test_strict_mention_string_true():
+    adapter = _make_adapter(strict_mention="true")
+    assert adapter._slack_strict_mention() is True
+
+
+def test_strict_mention_string_off():
+    adapter = _make_adapter(strict_mention="off")
+    assert adapter._slack_strict_mention() is False
+
+
+def test_strict_mention_malformed_stays_false():
+    """Unrecognised values keep strict mode OFF (fail-open to legacy behavior)."""
+    adapter = _make_adapter(strict_mention="maybe")
+    assert adapter._slack_strict_mention() is False
+
+
+def test_strict_mention_env_var_fallback(monkeypatch):
+    monkeypatch.setenv("SLACK_STRICT_MENTION", "true")
+    adapter = _make_adapter()  # no config value -> falls back to env
+    assert adapter._slack_strict_mention() is True
+
+
 # ---------------------------------------------------------------------------
 # Tests: _slack_free_response_channels
 # ---------------------------------------------------------------------------
@@ -350,3 +394,24 @@ def test_config_bridges_slack_reply_in_thread(monkeypatch, tmp_path):
         reply_to="171.500",
         metadata={"thread_id": "171.000"},
     ) == "171.000"
+
+
+def test_config_bridges_slack_strict_mention(monkeypatch, tmp_path):
+    from gateway.config import load_gateway_config
+
+    hermes_home = tmp_path / ".hermes"
+    hermes_home.mkdir()
+    (hermes_home / "config.yaml").write_text(
+        "slack:\n"
+        "  strict_mention: true\n",
+        encoding="utf-8",
+    )
+
+    monkeypatch.setenv("HERMES_HOME", str(hermes_home))
+    monkeypatch.delenv("SLACK_STRICT_MENTION", raising=False)
+
+    config = load_gateway_config()
+
+    assert config is not None
+    import os as _os
+    assert _os.environ["SLACK_STRICT_MENTION"] == "true"