refactor: replace :CP login with :CP credentials subcommand

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.

lua/cp/login.lua

@@ -1,32 +0,0 @@
-local M = {}
-
-local cache = require('cp.cache')
-local logger = require('cp.log')
-local state = require('cp.state')
-
-function M.login(platform)
-  platform = platform or state.get_platform()
-  if not platform then
-    logger.log('No platform specified. Usage: :CP login <platform>', vim.log.levels.ERROR)
-    return
-  end
-
-  vim.ui.input({ prompt = platform .. ' username: ' }, function(username)
-    if not username or username == '' then
-      logger.log('Login cancelled', vim.log.levels.WARN)
-      return
-    end
-    vim.fn.inputsave()
-    local password = vim.fn.inputsecret(platform .. ' password: ')
-    vim.fn.inputrestore()
-    if not password or password == '' then
-      logger.log('Login cancelled', vim.log.levels.WARN)
-      return
-    end
-    cache.load()
-    cache.set_credentials(platform, { username = username, password = password })
-    logger.log(platform .. ' credentials saved', vim.log.levels.INFO)
-  end)
-end
-
-return M