refactor: replace :CP login with :CP credentials subcommand

Problem: :CP login was a poor API — no way to clear credentials without
raw Lua, and the single command didn't scale to multiple operations.

Solution: replace login with a :CP credentials subcommand following the
same pattern as :CP cache. :CP credentials set [platform] prompts and
saves; :CP credentials clear [platform] removes one or all platforms.
Add cache.clear_credentials(), rename login.lua to credentials.lua,
update parse/dispatch/tab-complete, and rewrite vimdoc accordingly.

lua/cp/constants.lua

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