## 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).