[frontend-contrib S24] Convention generator (generate frontend)

[rickylabs]

Part of #922

Scope

Phase-2 sugar: derive manifest contribution lists from the frontend/ file tree (explicit manifest stays the contract; export-map maintenance already owned by plugin dev).

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

Acceptance

• gate: generated lists byte-match a hand-written manifest for the dogfood plugin