feat(competition): round-1 challenger kit restoring the auto backend

[plind-junior]

round-1 ladder entry: restores backend: auto over the reigning canary's substring.

the substring backend scores ~0.2-0.4 composite below auto on any seed (verified locally over the day's paired seeds), so this should clear the dethrone band decisively. kit-only change; the gate scores it and, on a dethrone against the ladder branch, arms auto-merge — this pr is the live round-1 test of that full chain, including the koth-ledger row append after the merge.

reproduce: python .github/scripts/koth_score.py --champion <ladder kit> --challenger <this kit> --base-sha <ladder sha>

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[github-actions]

koth ladder - dethroned

{
 "date": "2026-07-27",
 "base_sha": "0ee9c5bd95c42d4e252673973821bc0f77c311c0",
 "seeds": [
  159638922805791,
  63915481777079,
  111542822922051,
  101949209528365,
  112935885919949,
  50507605821269,
  120837154012398,
  39444906556475,
  181417274873639,
  130345037020083,
  167615437404411,
  240019499830169
 ],
 "lane": "kit",
 "champion": {
  "scores": [
   0.2,
   0.2,
   0.2,
   0.2,
   0.2,
   0.2,
   0.2,
   0.2,
   0.2,
   0.2,
   0.2,
   0.2
  ],
  "mean": 0.2
 },
 "challenger": {
  "scores": [
   0.7,
   0.5,
   0.5,
   0.3,
   0.5,
   0.5,
   0.6,
   0.6,
   0.5,
   0.5,
   0.6,
   0.6
  ],
  "mean": 0.5333333333333333
 },
 "mean_diff": 0.3333333333333333,
 "se": 0.028426762180748057,
 "band": 0.05571645387426619,
 "dethroned": true
}

auto-merge on dethrone (provisional ladder branch).

seeds derive from base sha + utc date; rerun locally with
python .github/scripts/koth_score.py --date <utc-date> to reproduce.
the daily result is provisional and overfittable - payout rank is
settled monthly on sealed commit-reveal seeds (docs/vouchbench-seasons.md).