docs: M3-backend Plan A implementation plan + spec refinements
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>
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