docs: wire define-product layer

[aryeko]

Summary

Wires the org docs to M3, the Product-layer PRD and acceptance-criteria contract, while preserving the existing local M1/M2 bookkeeping edits.

What M3 owns

M3 owns the PRD and stable acceptance-criteria-ID seam through the new define-product repo. Product owns outcome, user job, acceptance criteria, constraints, assumptions, non-goals, ID stability, and citation rules.

Why this is Product, not Planning

The roadmap and milestones point downstream layers at the Product contract as a cited input. They do not add design-to-plan mechanics or Planning-layer implementation scope.

Org docs wired

• Updates ROADMAP.md to name define-product as the Product layer and PRD seam owner.
• Updates MILESTONES.md so M3's owner and artifacts point at define-product.
• Updates profile/README.md so the org landing page lists define-product and marks M3 as current focus.
• Preserves the parent-thread M1/M2 edits marking Jig contract design and Technical Design handoff complete.

Verification

• pnpm format
• pnpm check

[aryeko]

Review findings before M3 merge:

1. ROADMAP.md says "Two stages already exist as repos" while listing define-product, technical-design, and jig. That is three repos and makes the org status summary internally inconsistent.

2. The Planning layer section and "What can start in parallel now" still say Planning only depends on Jig's execution-plan schema and technical-design's document format. After M3, Planning also consumes the Product PRD / acceptance-criteria-ID contract; this should be reflected here so M4 is not framed as independent of M3 while MILESTONES.md correctly requires M3 to be pinned.

Please fix those consistency issues, rerun pnpm check, and push the PR branch.