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). Update
parser, dispatcher, tab completion, and vimdoc.
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.
