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.

website/docs/user-guide/messaging/slack.md

@@ -264,6 +264,22 @@ For backward compatibility with older manifests, you can still type
 run the tests`. Free-form questions also work: `/hermes what's the
 weather?` is treated as a regular message.
 
+### Using commands inside threads (the `!cmd` prefix)
+
+Slack itself blocks native slash commands inside thread replies — try
+`/queue` in a thread and Slack responds with *"/queue is not supported
+in threads. Sorry!"* There is no app-side setting that re-enables them;
+Slack never delivers them to Hermes.
+
+As a workaround, Hermes recognises a leading `!` as an alternate
+command prefix that works in threads (and anywhere else). Type
+`!queue`, `!stop`, `!model gpt-5.4`, etc. as a regular thread reply —
+Hermes treats it identically to the slash form and replies in the same
+thread.
+
+Only the first token is checked against the known command list, so
+casual messages like `!nice work` pass through to the agent unchanged.
+
 ### Advanced: emit only the slash-commands array
 
 If you maintain your Slack manifest by hand and just want the slash