Same 5-step preflight body (setup-bun, setup-node, cache, install,
lint, format:check) was duplicated across 5 workflows. Move it to
.github/actions/preflight/action.yml so future tweaks (adding
typecheck, swapping the cache key, etc.) are a single-file change.
Net diff: +33 / -65.
Addresses the "shared preflight" follow-up Vai called out on #877.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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