feat: add :CP login command for explicit credential management

Problem: credentials were only set implicitly on first :CP submit.
There was no way to update wrong credentials, log out, or set
credentials ahead of time without editing the cache JSON manually.

Solution: add :CP login [platform] which always prompts for username
and password and overwrites any saved credentials for that platform.
Omitting the platform falls back to the active platform. Wire the
command through constants, parse_command, handle_command, and add
tab-completion (suggests platform names). Document in vimdoc under
the SUBMIT section and in the commands reference.

lua/cp/constants.lua

@@ -13,6 +13,7 @@ M.ACTIONS = {
   'race',
   'stress',
   'submit',
+  'login',
 }
 
 M.PLATFORM_DISPLAY_NAMES = {