#1583 Docs: document catalog completeness check and --repair in doctor reference

Shipping with #1580: swamp doctor datastores gains a catalog_completeness health check, and --repair gains the ability to rebuild the local catalog index. content/manual/reference/doctor.md describes neither.

What needs covering:

• The check compares the local catalog index against the on-disk data walk and reports any model type whose records are on disk but missing from the index.
• It runs on every repo, including ones with no namespace and no remote datastore configured — unlike the namespace migration and contamination checks.
• swamp doctor datastores --repair previews; --repair -y removes _catalog.db so the next data query rebuilds it.
• Caveat worth stating explicitly: the rebuild also clears foreign-namespace rows fetched by swamp datastore catalog pull, because those describe data not on local disk and a local walk cannot recreate them. Re-pull to restore.
• Behavior change: swamp doctor datastores --repair on a repo with no custom datastore or namespace previously errored with "Repair is only available for custom datastores with a namespace configured". It now runs the catalog repair and skips the contamination repair.

Symptom worth documenting so affected users can find this: queries return nothing for a model whose data is visibly present via swamp data get / swamp data list.

Filed here rather than on the docs repo, which has GitHub issues disabled.