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/credentials.lua

@@ -0,0 +1,45 @@
+local M = {}
+
+local cache = require('cp.cache')
+local logger = require('cp.log')
+local state = require('cp.state')
+
+function M.set(platform)
+  platform = platform or state.get_platform()
+  if not platform then
+    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('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('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, 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