refactor(race): replace :CP race subcommand with --race flag

Problem: Race was the only command using `:CP <action> <platform> <contest>`.
Every other platform+contest command uses `:CP <platform> <contest> [flags]`.

Solution: Make race a flag on contest setup: `:CP <platform> <contest> --race`.
Remove `:CP race stop` — starting a new race auto-cancels any active one.
Remove `<Plug>(cp-race-stop)` keymap. Update tab completion for `--race`.

lua/cp/constants.lua

@@ -10,7 +10,6 @@ M.ACTIONS = {
   'cache',
   'interact',
   'edit',
-  'race',
   'stress',
   'submit',
   'open',