docs(overlay): require parent-authored judge briefs

[zhanfeng-alpha]

Why

Keep the judge-brief rule specific to Pi. Prefer a parent-inline pick when context allows. If a judge child is needed, the parent authors its task with hard constraints, an explicit preference order, and "Do not invent parent preference."

Scope

Only overlay/AGENTS.md changes. The short rule limits judge file access to listed design/*.md paths and keeps the final pick with the parent.

No installer, architect, poteto-mode, upstream pstack, or CI changes. The earlier install-time adaptation and its tests were removed from this PR.

Verification

• All 18 existing Python tests passed.
• Local overlay, skill-conformance, and whitespace checks passed.
• A temporary installation contains the overlay rule. Architect and poteto-mode match baseline conformance, and their upstream source hashes remain unchanged.

[zhanfeng-alpha]

Superseded by #13. The A+C overlays remove the mandatory competing-design and judge workflow, permit parent-owned single-model sketches, and reapply that policy on install and refresh. The narrower judge-brief rule is no longer needed.