PROV-O JSON-LD export for the publish bundle (deferred: no current verifier)

Migrated from: https://github.com/jeremi/registry-manifest/issues/17
Original author: @jeremi
Source repository: `jeremi/registry-manifest`
Source issue: `#17`
Source labels: post-1.0
Source milestone/release intent: _none_
Target area/path: `products/manifest`

## Issue

[redacted private/internal deployment detail during migration]

Manifest is the cleanest PROV-O candidate in the stack: publish already produces discrete artifacts, digests, a generation activity, and publisher metadata, mapping directly to `prov:Entity`/`prov:Activity`/`prov:Agent`. The concept note specifies the mapping table, an optional `provenance.jsonld` output, golden fixtures, and the circular-digest exclusion rule.

Deferred because no verifier, operator, or interoperability partner consumes PROV-O today, and the artifact is additive, so adding it later breaks nothing (guaranteed by the extension-policy beta work). Pick this up when a concrete consumer appears.

_Note: one or more private/internal references were redacted during migration._

## Migration Metadata

- Migrated to the public monorepo on 2026-06-25.
- Source title, body, labels, milestone/release intent, and pre-migration discussion were preserved where available.
- Code-grounded audit note: PROV-O export is still documented as not implemented; CLI only tolerates injected provenance.
- Private/internal references, secrets, and obvious deployment-only details were redacted instead of copied forward.

When a concrete consumer appears, start with the concept note and the products/manifest area, then review the publish bundle's existing artifacts, digests, generation activity, and publisher metadata. Use the specified mapping table and golden fixtures as the guide; done means an optional provenance.jsonld artifact is produced while applying the circular-digest exclusion rule.

Written by the indexing model from the issue text.