## Problem
httpx scrapers (CSES, Kattis, USACO) always ran full login flows even
with valid cached sessions. \`credentials.lua\` always prompted before
trying cached credentials. \`default_filename\` doubled the slug for
Kattis single-problem mode (e.g. \`addtwonumbersaddtwonumbers.cc\`).
## Solution
Added token/cookie fast paths to CSES \`login()\` and USACO
\`login()\`/\`submit()\`.
Hardened Kattis reactive re-auth trigger to check status code first.
Refactored \`credentials.lua\` to try cached credentials before
prompting.
Fixed \`default_filename\` to not concatenate when \`contest_id ==
problem_id\`.
564f9286da