#3381 Registry UI and manual say Behaviors, keeping the stored models content type

Summary

Follow-up to the behavior series (swamp-club#3329–#3334, #3338). "Behavior" is now the product term for what swamp called a model. The swamp CLI, dashboards and skills use it; the swamp-club registry UI and manual still say "Models".

No registry code change is required: the CLI keeps sending contentMetadata.models, the archive models/ dir and the models: manifest key, and a trace of swamp-club confirmed nothing server-side matches export names. This issue is the display and docs change only.

Scope

• Registry UI labels: the "Models" section on the extension page (routes/extensions/[...name].tsx, ModelCard), the section nav label (lib/app/views/extension-view.ts), the content-type facet label in islands/ExtensionRegistry.tsx (and the hero copy "models, drivers, vaults, and reports"), the "MODELS" chip in components/ExtensionList.tsx, and the version-diff category label.
• Keep the stored and wire value contentType: models unchanged, so ?contentType=models URLs, stored facet data and older CLIs keep working. Only the displayed label changes.
• Manual: content/manual/how-to/extensions/create-and-publish.md and use-local.md teach extensions/models and export const model. Update them to the behavior spelling (export const behavior, extensions/behaviors/, behaviors:), with a note that the model spelling still works and that clients older than the release with swamp-club#3338 cannot load behavior-spelled extensions. Also update the swamp-serve dashboard reference (URLs are now /dashboard/behaviors/..., old /dashboard/models/... links redirect) and any CLI reference pages to swamp behavior ... commands.
• Check tests/content/terminology_test.ts rules for conflicts with the new wording.

Acceptance

• Registry pages and facets say Behaviors; ?contentType=models links still filter correctly.
• Manual pages teach the behavior spelling with the compatibility note.