feat(slack): support !cmd as alternate prefix for slash commands in threads (#25355)

Slack platform-blocks native slash commands inside thread replies ("/queue
is not supported in threads. Sorry!") and there is no app-side setting to
re-enable them. As a workaround, rewrite a leading '!' to '/' for any known
gateway command before downstream processing — so '!queue', '!stop',
'!model gpt-5.4' etc. work inside Slack threads (and anywhere else).

Only the first token is checked against is_gateway_known_command(), so
casual messages like '!nice work' pass through to the agent unchanged.
Downstream pipeline (MessageType.COMMAND tagging, gateway dispatcher,
thread reply routing) is unchanged.

Adds 6 tests covering rewrite, args preservation, thread routing,
casual-message passthrough, '@bot' suffix, and plain '/' still-works.

tests/gateway/test_slack.py

@@ -691,10 +691,98 @@ class TestSendVideo:
         adapter._app.client.chat_postMessage.assert_called_once()
 
 
+# ---------------------------------------------------------------------------
+# TestBangPrefixCommands
+# ---------------------------------------------------------------------------
+
+
+class TestBangPrefixCommands:
+    """``!cmd`` is rewritten to ``/cmd`` so commands work inside Slack threads.
+
+    Slack natively rejects slash commands invoked from a thread reply
+    ("/queue is not supported in threads. Sorry!"). Typing ``!queue`` as a
+    plain text reply hits the message event pipeline instead, and the
+    adapter rewrites the leading ``!`` to ``/`` for any known gateway
+    command before downstream processing.
+    """
+
+    def _make_event(self, text, thread_ts=None, channel_type="im", channel="D123"):
+        evt = {
+            "text": text,
+            "user": "U_USER",
+            "channel": channel,
+            "channel_type": channel_type,
+            "ts": "1234567890.000001",
+        }
+        if thread_ts:
+            evt["thread_ts"] = thread_ts
+        return evt
+
+    @pytest.mark.asyncio
+    async def test_bang_known_command_is_rewritten_to_slash(self, adapter):
+        """``!queue`` → ``/queue`` and tagged as COMMAND."""
+        await adapter._handle_slack_message(self._make_event("!queue"))
+
+        adapter.handle_message.assert_called_once()
+        msg_event = adapter.handle_message.call_args[0][0]
+        assert msg_event.text.startswith("/queue")
+        assert msg_event.message_type == MessageType.COMMAND
+
+    @pytest.mark.asyncio
+    async def test_bang_command_with_args_preserved(self, adapter):
+        """``!model gpt-5.4`` → ``/model gpt-5.4``."""
+        await adapter._handle_slack_message(self._make_event("!model gpt-5.4"))
+
+        msg_event = adapter.handle_message.call_args[0][0]
+        assert msg_event.text.startswith("/model gpt-5.4")
+        assert msg_event.message_type == MessageType.COMMAND
+
+    @pytest.mark.asyncio
+    async def test_bang_works_inside_thread(self, adapter):
+        """The whole point: ``!stop`` inside a thread reply dispatches."""
+        evt = self._make_event("!stop", thread_ts="1111111111.000001")
+        await adapter._handle_slack_message(evt)
+
+        msg_event = adapter.handle_message.call_args[0][0]
+        assert msg_event.text.startswith("/stop")
+        assert msg_event.message_type == MessageType.COMMAND
+        # thread_id is preserved on the source so the reply lands in the
+        # same thread.
+        assert msg_event.source.thread_id == "1111111111.000001"
+
+    @pytest.mark.asyncio
+    async def test_bang_unknown_token_passes_through_unchanged(self, adapter):
+        """``!nice work`` is just a casual message — must NOT be rewritten."""
+        await adapter._handle_slack_message(self._make_event("!nice work"))
+
+        msg_event = adapter.handle_message.call_args[0][0]
+        assert msg_event.text == "!nice work"
+        assert msg_event.message_type != MessageType.COMMAND
+
+    @pytest.mark.asyncio
+    async def test_bang_with_bot_suffix_resolves(self, adapter):
+        """``!stop@hermes`` matches the get_command() ``@suffix`` stripping."""
+        await adapter._handle_slack_message(self._make_event("!stop@hermes"))
+
+        msg_event = adapter.handle_message.call_args[0][0]
+        assert msg_event.text.startswith("/stop@hermes")
+        assert msg_event.message_type == MessageType.COMMAND
+
+    @pytest.mark.asyncio
+    async def test_plain_slash_still_works(self, adapter):
+        """Sanity check — ``/queue`` (top-level channel/DM) still dispatches."""
+        await adapter._handle_slack_message(self._make_event("/queue"))
+
+        msg_event = adapter.handle_message.call_args[0][0]
+        assert msg_event.text.startswith("/queue")
+        assert msg_event.message_type == MessageType.COMMAND
+
+
 # ---------------------------------------------------------------------------
 # TestIncomingDocumentHandling
 # ---------------------------------------------------------------------------
 
+
 class TestIncomingDocumentHandling:
     def _make_event(self, files=None, text="hello", channel_type="im", blocks=None, attachments=None):
         """Build a mock Slack message event with file attachments."""