ci(repo): consolidate mutation runners and gate the score - #140

What

Reworks .github/workflows/mutation.yml so the ~25 mutation packages run on a handful of runner machines instead of one per package, while keeping both incremental mechanisms and making the mutation score gate PRs.

Changes

• Bucket the matrix (discover chunks sorted packages into groups of 5, emitting buckets + package_count; mutation matrix iterates buckets, running turbo mutation with --concurrency=2 --continue=dependencies-successful).
• Warm the turbo cache — restore/save .turbo/cache under a mutation-specific per-bucket key (turbo-mutation-<os>-<bucket>-<sha>) so it never collides with the check job's turbo-... key on the same commit.
• Keep per-package incremental reports — staged per bucket under bucket-incremental/, re-keyed to stryker-incremental-<bucket>-main.
• Gate the score — removed continue-on-error: true from the mutation step, so a package below its break threshold fails the check (reverses the prior advisory posture).
• Merge across buckets — scripts/tools/merge-mutation-reports.mjs readParts now discovers parts recursively so the aggregate report spans every bucket.
• Doctrine — AGENTS.md REPO-D1 updated: CI's Mutation workflow now gates on the score too.

Decisions (settled with user)

• Fewer runners over worst-case wall-clock; dynamic runner count derived from package count (packages-per-bucket ratio).
• Full-coverage merged report (spans every package), not changed-only.
• Absolute-score gating: a package below its break threshold fails the check.

Verification

• pnpm check:local exits 0 (run after the rebase onto main).
• pnpm check:turbo-graph clean; guard-action-provenance clean.
• merge-mutation-reports.mjs --selftest passes (8 fixtures); recursive readParts smoke-tested against a synthetic 2-bucket/3-package layout (merges 3 of 3, aggregate **all** row at 100.00).
• turbo mutation --filter=./packages/oxlint-plugins/effect-store --dry=json confirms the volatile reports/stryker-incremental.json input is gone (0 incremental files in inputs).