feat(gateway/slack): add SLACK_REACTIONS env toggle for reaction lifecycle

Adds _reactions_enabled() gating to match Discord (DISCORD_REACTIONS) and
Telegram (TELEGRAM_REACTIONS) pattern. Defaults to true to preserve existing
behavior. Gates at three levels:
- _handle_slack_message: skips _reacting_message_ids registration
- on_processing_start: early return
- on_processing_complete: early return

Also adds config.yaml bridge (slack.reactions) and two new tests.

gateway/config.py

@@ -616,6 +616,8 @@ def load_gateway_config() -> GatewayConfig:
                     if isinstance(frc, list):
                         frc = ",".join(str(v) for v in frc)
                     os.environ["SLACK_FREE_RESPONSE_CHANNELS"] = str(frc)
+                if "reactions" in slack_cfg and not os.getenv("SLACK_REACTIONS"):
+                    os.environ["SLACK_REACTIONS"] = str(slack_cfg["reactions"]).lower()
 
             # Discord settings → env vars (env vars take precedence)
             discord_cfg = yaml_cfg.get("discord", {})

gateway/platforms/slack.py

@@ -607,8 +607,14 @@ class SlackAdapter(BasePlatformAdapter):
             logger.debug("[Slack] reactions.remove failed (%s): %s", emoji, e)
             return False
 
+    def _reactions_enabled(self) -> bool:
+        """Check if message reactions are enabled via config/env."""
+        return os.getenv("SLACK_REACTIONS", "true").lower() not in ("false", "0", "no")
+
     async def on_processing_start(self, event: MessageEvent) -> None:
         """Add an in-progress reaction when message processing begins."""
+        if not self._reactions_enabled():
+            return
         ts = getattr(event, "message_id", None)
         if not ts or ts not in self._reacting_message_ids:
             return
@@ -618,6 +624,8 @@ class SlackAdapter(BasePlatformAdapter):
 
     async def on_processing_complete(self, event: MessageEvent, outcome: ProcessingOutcome) -> None:
         """Swap the in-progress reaction for a final success/failure reaction."""
+        if not self._reactions_enabled():
+            return
         ts = getattr(event, "message_id", None)
         if not ts or ts not in self._reacting_message_ids:
             return
@@ -1260,7 +1268,7 @@ class SlackAdapter(BasePlatformAdapter):
         # Only react when bot is directly addressed (DM or @mention).
         # In listen-all channels (require_mention=false), reacting to every
         # casual message would be noisy.
-        _should_react = is_dm or is_mentioned
+        _should_react = (is_dm or is_mentioned) and self._reactions_enabled()
         if _should_react:
             self._reacting_message_ids.add(ts)
 

tests/gateway/test_slack.py

@@ -1138,6 +1138,56 @@ class TestReactions:
         adapter._app.client.reactions_add.assert_not_called()
         adapter._app.client.reactions_remove.assert_not_called()
 
+    @pytest.mark.asyncio
+    async def test_reactions_disabled_via_env(self, adapter, monkeypatch):
+        """SLACK_REACTIONS=false should suppress all reaction lifecycle."""
+        monkeypatch.setenv("SLACK_REACTIONS", "false")
+        adapter._app.client.reactions_add = AsyncMock()
+        adapter._app.client.reactions_remove = AsyncMock()
+        adapter._app.client.users_info = AsyncMock(return_value={
+            "user": {"profile": {"display_name": "Tyler"}}
+        })
+
+        event = {
+            "text": "hello",
+            "user": "U_USER",
+            "channel": "C123",
+            "channel_type": "im",
+            "ts": "1234567890.000004",
+        }
+        await adapter._handle_slack_message(event)
+
+        # Should NOT register for reactions when toggle is off
+        assert "1234567890.000004" not in adapter._reacting_message_ids
+
+        # Hooks should also be no-ops when disabled
+        from gateway.platforms.base import MessageEvent, MessageType, SessionSource, ProcessingOutcome
+        from gateway.config import Platform
+        source = SessionSource(
+            platform=Platform.SLACK,
+            chat_id="C123",
+            chat_type="dm",
+            user_id="U_USER",
+        )
+        msg_event = MessageEvent(
+            text="hello",
+            message_type=MessageType.TEXT,
+            source=source,
+            message_id="1234567890.000004",
+        )
+        # Force-add to verify hooks respect the toggle independently
+        adapter._reacting_message_ids.add("1234567890.000004")
+        await adapter.on_processing_start(msg_event)
+        await adapter.on_processing_complete(msg_event, ProcessingOutcome.SUCCESS)
+
+        adapter._app.client.reactions_add.assert_not_called()
+        adapter._app.client.reactions_remove.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_reactions_enabled_by_default(self, adapter):
+        """SLACK_REACTIONS defaults to true (matches existing behavior)."""
+        assert adapter._reactions_enabled() is True
+
 
 # ---------------------------------------------------------------------------
 # TestThreadReplyHandling