Implementation plan for spec phases 1-3: scaffold five core-* packages,
populate core-shared with Payload primitives + tRPC plumbing, and lift
the Payload config into a stub core-cms with apps/cms repointed at it.
First of four plans in the vertical-refactor sequence.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
