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/cache.lua

@@ -388,6 +388,18 @@ function M.set_credentials(platform, creds)
   M.save()
 end
 
+---@param platform string?
+function M.clear_credentials(platform)
+  if platform then
+    if cache_data._credentials then
+      cache_data._credentials[platform] = nil
+    end
+  else
+    cache_data._credentials = nil
+  end
+  M.save()
+end
+
 function M.clear_all()
   local creds = cache_data._credentials
   cache_data = {}

lua/cp/commands/init.lua

@@ -83,12 +83,20 @@ local function parse_command(args)
       else
         return { type = 'action', action = 'interact' }
       end
-    elseif first == 'login' then
-      return {
-        type = 'action',
-        action = 'login',
-        platform = args[2],
-      }
+    elseif first == 'credentials' then
+      local subcommand = args[2]
+      if not subcommand then
+        return { type = 'error', message = 'credentials command requires subcommand (set, clear)' }
+      end
+      if vim.tbl_contains({ 'set', 'clear' }, subcommand) then
+        return {
+          type = 'credentials',
+          subcommand = subcommand,
+          platform = args[3],
+        }
+      else
+        return { type = 'error', message = 'unknown credentials subcommand: ' .. subcommand }
+      end
     elseif first == 'stress' then
       return {
         type = 'action',
@@ -323,8 +331,6 @@ function M.handle_command(opts)
       edit.toggle_edit(cmd.test_index)
     elseif cmd.action == 'stress' then
       require('cp.stress').toggle(cmd.generator_cmd, cmd.brute_cmd)
-    elseif cmd.action == 'login' then
-      require('cp.login').login(cmd.platform)
     elseif cmd.action == 'submit' then
       require('cp.submit').submit({ language = cmd.language })
     elseif cmd.action == 'race' then
@@ -360,6 +366,13 @@ function M.handle_command(opts)
 
     local setup = require('cp.setup')
     setup.setup_contest(platform, contest_id, problem_id, cmd.language)
+  elseif cmd.type == 'credentials' then
+    local creds = require('cp.credentials')
+    if cmd.subcommand == 'set' then
+      creds.set(cmd.platform)
+    elseif cmd.subcommand == 'clear' then
+      creds.clear(cmd.platform)
+    end
   elseif cmd.type == 'cache' then
     local cache_commands = require('cp.commands.cache')
     cache_commands.handle_cache_command(cmd)

lua/cp/constants.lua

@@ -13,7 +13,7 @@ M.ACTIONS = {
   'race',
   'stress',
   'submit',
-  'login',
+  'credentials',
 }
 
 M.PLATFORM_DISPLAY_NAMES = {

lua/cp/credentials.lua

@@ -4,29 +4,42 @@ local cache = require('cp.cache')
 local logger = require('cp.log')
 local state = require('cp.state')
 
-function M.login(platform)
+function M.set(platform)
   platform = platform or state.get_platform()
   if not platform then
-    logger.log('No platform specified. Usage: :CP login <platform>', vim.log.levels.ERROR)
+    logger.log(
+      'No platform specified. Usage: :CP credentials set <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)
+      logger.log('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)
+      logger.log('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)
+    logger.log(platform .. ' credentials saved', vim.log.levels.INFO, true)
   end)
 end
 
+function M.clear(platform)
+  cache.load()
+  cache.clear_credentials(platform)
+  if platform then
+    logger.log(platform .. ' credentials cleared', vim.log.levels.INFO, true)
+  else
+    logger.log('all credentials cleared', vim.log.levels.INFO, true)
+  end
+end
+
 return M