Adds a pinned Knip check to CI that fails on unused files, exports, or
dependencies — cross-module dead-code detection Biome's in-file noUnused* can't
do. Tuned to zero false positives on this library (release.config.js declared as
an entry). Pilot repo for the fleet dead-code rollout; expand to other TS repos
after it proves clean.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>