Surface the _provenance.json schema v2 as a renderable schema reference

> _Filed by Claude (AI assistant) on @jmchilton's behalf — not personally authored._

## Gap

The `_provenance.json` schema v2 contract is described inline in prose in `docs/COMPILATION_PIPELINE.md`. An external reader (or auditor) who wants to read the provenance contract has to hunt through narrative rather than read a schema. The Foundry already has a `type: schema` note convention with site rendering and `$defs` deep-linking — provenance is the obvious candidate that isn't using it.

## Proposed fix

Surface `_provenance.json` v2 as its own renderable schema reference, following the existing schema-note convention:
- a human-readable schema note under `content/schemas/` (per the established pattern: `package` / `package_export` pointer if a TS package owns the shape; otherwise document where the contract is enforced — `cast-skill-verify.ts` / the verifier);
- rendered on the site with field-level anchors so docs and Mold bodies can deep-link specific provenance fields;
- `COMPILATION_PIPELINE.md` keeps the narrative but links to the schema note as the contract of record.

## Constraints

- Follow the existing schema-note authoring rules (frontmatter, `meta_schema.yml`, registered tags, wiki-links). Don't invent a new mechanism — provenance should ride the same rails as `summary-nextflow` / `tests-format`.
- The schema is a versioned contract (v2 today); the note should make the version explicit and survive a future v3 deliberately.
- `npm run validate` + site build clean.

Effort: low. Surfaced during review of transient grant-proposal hardening notes.