docs: define PMO01 platform foundation and A to C strategy

[tim8es]

Summary

Formalizes PMO01's platform foundation before further development.

Decision

• Keep current main as a reference prototype.
• Validate learning mechanisms before architecture expansion.
• Build scalable v1 separately from validated product/domain contracts.
• Migrate vertically, starting with one validated module.

Added

• documentation map and precedence rules;
• product contract;
• learning model;
• curriculum competency contract;
• staged product requirements;
• learning/product metrics;
• canonical content model;
• current/target architecture;
• ADR 0001 for the A → C strategy;
• gated roadmap;
• platform-foundation design spec.

Changed

• root README now labels the current application as a reference prototype and points to the documentation source of truth.

Not changed

• no application code;
• no current prototype behavior;
• no framework migration;
• no backend/auth/AI/CMS work.

Review focus

Please review the platform foundation spec first:
docs/superpowers/specs/2026-09-06-pmo01-platform-foundation-design.md

After this foundation is accepted, the next plan should cover Phase 1 only: validate one representative module end-to-end. It should not start a full v1 rewrite yet.