feat: changed-file mutation gate as a second required PR check (D-027) - #12
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Task 10 rehearsal (scratch PR #11, closed without merging): concurrency 1 hit the 15-minute job timeout at a 409-line mutate set with no report produced; concurrency 4 completed the same set cleanly in 7.7 min at 27.5 mutants/min, with no coverage-correlation warnings in the log. Concurrency 4 ships via MUTATION_GATE_EXTRA_ARGS. Derived sustainable threshold (27.5 * 12 / 0.5, rounded) is 700 lines, within 2x of the existing 800 default, so thresholdLines stays unchanged. Also carries back two stryker.conf.json fixes the rehearsal found necessary for the gate to run in CI at all (not rehearsal-specific, so they belong here rather than staying scratch-only): - tsconfigFile pointed at a nonexistent path, sidestepping a crash in Stryker 9.6.1's TSConfigPreprocessor (ts.parseConfigFileTextToJson, removed from TypeScript 7's module surface per D-022) that fired on every invocation regardless of which files were mutated - bun.timeout raised to 120s so the dry run's full-suite pass (~68-90s observed in CI) survives the runner plugin's own internal process timeout, which otherwise killed it well before Stryker's own dryRunTimeoutMinutes ever applied
…re, review minors)
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Implements docs/superpowers/specs/2026-08-04-mutation-pr-gate-design.md: scripts/mutation-gate.mjs + .github/workflows/mutation.yml (the portable two-file unit), changed-file mode gating on zero undetected mutants, loud-skip over the size threshold with mutate-force/mutate-skip label overrides, incremental mode for adopters, report artifact on failure. Rehearsed end to end on a scratch PR (red on a planted survivor, green after the kill, both labels, skip-size, sticky comment upsert, artifact); concurrency and threshold set from the measured rate (D-027).
The rehearsal also surfaced two gate-blocking stryker.conf.json issues (fixed: tsconfigFile sentinel no-op for the TS7 crash, bun.timeout for the dry run) and one post-D-011 drift survivor at engine/index.ts:311 (killed; full engine recertified clean) — see D-027.