Brain/OKF: per-directory `index.md` (progressive disclosure)

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Part of the Brain ⇄ OKF compatibility effort (plan: docs/brain-okf-compat.md).

Why

OKF §6 defines an optional per-directory index.md (progressive disclosure: * [Title](rel) - description, no frontmatter), auto-generatable and synthesizable on the fly. Today the Brain has a single root MEMORY.md index per scope. Per-directory indexes scale better for deep/large trees (descend index→index instead of loading one giant file) — and it's exactly the Karpathy llm-wiki pattern cited in brain.md §4.5.

Scope

• The curator (brain.curate) generates/maintains an OKF-conformant index.md in each non-empty numbered folder.
• Idempotent + auto-synthesizable; root MEMORY.md stays the top index and points at the per-dir indexes (reconcile MEMORY.md ⇄ OKF index.md here).
• Read-order SKILL updated to descend index→index at scale.
• index.md is a reserved file (no frontmatter) — exclude from the type-conformance rule (#U-OKF-3).

Acceptance criteria

• After brain.curate, every non-empty durable folder has a conformant index.md (* [Title](rel) - desc, no frontmatter).
• Re-running curate is a no-op (idempotent).
• Root index declares okf_version (coordinated with #U-OKF-3).
• Unit + integration + live tests; no regressions.

Files (likely)

packages/@boringos/brain/src/curate.ts, memory/brain SKILL.md.

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Tracked in epic #73 (Brain ⇄ OKF compatibility).