P2: Decouple SSG from app model via protocol contracts

[SisyphusZheng]

What to build

packages/ssg/src/drivers.ts:16 imports createRouteGraph from @openelement/app/model. Define the needed contract in protocol and have app implement it, so SSG no longer depends on the product layer.

Acceptance criteria

• Protocol contract exists for route graph creation.
• ssg no longer imports from @openelement/app.
• All tests pass.

Blocked by

None