[frontend-contrib S13] plugin new --with frontend

[rickylabs]

Part of #922

Scope

Plugin-side skeleton: frontend/ tree, manifest block, seeded deno.json export map, frontend_test.ts calling defineFrontendTestSuite.

Design source: RFC #890 (rfc.md §5) + .llm/runs/plan-frontend-contrib--seed/design/canonical/ (rev 3).

Acceptance

• gate: scaffolded plugin passes generate + install + its own test suite unedited