refactor(credentials): promote login/logout to top-level actions (#292)

## 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 = {