refactor(race): replace :CP race with --race flag (#347)

## Problem

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

## Solution

Make race a `--race` flag on the existing contest setup flow. Remove
`:CP race stop` — starting a new race auto-cancels any active one.
Remove `<Plug>(cp-race-stop)` keymap. Update tab completion and docs
accordingly.

lua/cp/constants.lua

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