feat(export): OKF re-export honesty — reproduce foreign frontmatter

[ethanj]

What

Completes the OKF round-trip so that a page imported from a foreign bundle re-exports honestly rather than as a generic concept.

Import captures an imported document's original frontmatter under an x-okf snapshot (an unknown type such as BigQuery Table maps to kind: concept while the raw type and any producer-specific keys are preserved). Until now nothing read that snapshot back, so re-export emitted type: concept and dropped foreign keys. This wires the snapshot through and re-exports with the right contract:

• ExportPage carries an optional xOkf snapshot; collect reads the x-okf block back from page frontmatter.
• On re-export of a page that has a snapshot, mapPageToOkfFrontmatter:
  • preserves the raw foreign type and any unknown producer keys (e.g. vendorKey);
  • derives the OKF standard fields (title, description, tags, timestamp) from the current page, so local edits after import are reflected rather than publishing stale snapshot values;
  • refreshes the x-llmwiki provenance block (content hash, page directory, provenance) from the current page;
  • does not emit x-okf into the exported OKF doc (it stays a llmwiki-side record).
• Native pages (no snapshot) map exactly as before.
• The original bundle path is persisted durably under x-okf.okfPath at import, so a page's origin survives review approval and is surfaced by collectExportPages (previously it lived only on the review candidate and was lost once approved).

Why

A foreign bundle should survive import→export honestly — preserving the producer's own type and keys while still reflecting any local edits and layering llmwiki's own provenance. This lets llmwiki act as a faithful pass-through for third-party OKF content.

Scope / known limitation

Re-export places every document at <pageDirectory>/<slug>.md — the llmwiki slug is the identity the OKF link rewriter and index TOC understand (/concepts|queries/<slug>.md). A foreign tables/customers.md therefore re-exports as concepts/tables-customers.md; its original path is preserved durably under x-okf.okfPath for diagnosis. Faithful reconstruction of nested original bundle paths is intentionally deferred — it requires extending the slug-based link/index model.

Test plan

• npx tsc --noEmit, npm run build, npm test (1619 passed), npx fallow (0 above threshold), dupes clean
• Unknown foreign type + producer key survive import→export
• Editing an imported page's title/summary before re-export publishes the current values while still preserving type: BigQuery Table + vendorKey
• x-okf does not leak into exported OKF frontmatter
• okfPath persists on the live page across real review approve and is surfaced by export
• Citation idempotence: export→import→export yields exactly one regenerated # Citations
• Native pages (no snapshot) map unchanged