Adds the bite-sized TDD implementation plan for Plan A (registry mirror +
runtime-config APIs + initial_state/unreviewed semantics; additive only).
Refines the spec with two planning discoveries: the re-stamp must be a
Python startup step (pure SQL can't read DEFAULT_PROJECT_ID), and the
PK-rebuild blast radius justifies splitting M3-backend into Plan A (this)
and Plan B (rebuild + project_id threading + re-stamp, before M4).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Ben Stull