fix(security): harden credential storage and transmission

Problem: credential and cookie files were world-readable (0644),
passwords transited via `CP_CREDENTIALS` env var (visible in
`/proc/PID/environ`), and Kattis/USACO echoed passwords back
through stdout unnecessarily.

Solution: set 0600 permissions on `cp-nvim.json` and `cookies.json`
after every write, pass credentials via stdin pipe instead of env
var, and stop emitting passwords in ndjson from Kattis/USACO
`LoginResult` (CSES token emission unchanged).

lua/cp/credentials.lua

@@ -117,6 +117,7 @@ function M.logout(platform)
     if ok and type(data) == 'table' then
       data[platform] = nil
       vim.fn.writefile({ vim.fn.json_encode(data) }, cookie_file)
+      vim.fn.setfperm(cookie_file, 'rw-------')
     end
   end
   logger.log(display .. ' credentials cleared', { level = vim.log.levels.INFO, override = true })