refactor(credentials): promote login/logout to top-level actions

Problem: :CP credentials login/logout/clear is verbose and inconsistent
with other actions that are all top-level (:CP run, :CP submit, etc.).
The clear-all subcommand is also unnecessary since re-logging in
overwrites existing credentials.

Solution: replace :CP credentials {login,logout,clear} with :CP login
[platform] and :CP logout [platform]. Remove the clear-all command and
the credentials subcommand dispatch — login/logout are now regular
actions routed through the standard action dispatcher.

lua/cp/constants.lua

@@ -13,7 +13,8 @@ M.ACTIONS = {
   'race',
   'stress',
   'submit',
-  'credentials',
+  'login',
+  'logout',
 }
 
 M.PLATFORM_DISPLAY_NAMES = {