## Problem
The `set` and `clear` subcommands don't clearly convey their intent —
`set`
reads like a generic setter rather than an auth action, and `clear`
overloads
single-platform and all-platform semantics in one subcommand.
## Solution
Rename `set` to `login`, split `clear` into `logout` (per-platform,
defaults
to active) and `clear` (all platforms).
New API:
- `:CP credentials login [platform]` — prompt and save credentials
- `:CP credentials logout [platform]` — remove credentials for one
platform
- `:CP credentials clear` — remove all stored credentials
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.
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.
Add command parsing and dispatch for :CP race, :CP race stop, :CP stress,
and :CP submit. Add tab-completion for race (platform/contest/--lang),
stress (cwd executables at arg 2 and 3), and race stop. Add
<Plug>(cp-stress), <Plug>(cp-submit), and <Plug>(cp-race-stop) keymaps.
