Brain/OKF: OKF bundle export (interop) + docs

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

Why

The payoff of OKF compatibility is interop: a scope's memory tree exported as a portable, vendor-neutral OKF bundle (readable by Obsidian/Notion/MkDocs/Dataplex/the OKF graph viewer, or re-importable elsewhere). This proves we are a true OKF superset and gives the Brain an export/exchange surface.

Scope

• brain.export_okf tool: emit a scope's memory tree as an OKF-conformant bundle (directory and/or tarball), generating any missing index.md, an okf_version, and an optional bundle-level log.md synthesized from 70-weekly/.
• Validate the emitted bundle against OKF §9 conformance before returning.
• Docs: update brain-implementation.md + brain README.md to state "files tier is an OKF superset"; add an OKF section to the public strategy doc.
• Stretch: brain.import_okf to ingest an external OKF bundle into a scope (indexer + graph wiring).

Acceptance criteria

• brain.export_okf produces a bundle that passes §9 conformance.
• The exported bundle round-trips back through the indexer (no data loss of type/links/citations).
• Docs updated; live (HTTP) export verifier; no regressions.

Files (likely)

packages/@boringos/brain/src/okf.ts (new), packages/@boringos/core/src/modules/brain.ts, scripts/verify-okf.mjs, docs.

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