notion-md: specify database/data-source workspace materialization

Context\n\nPR #679 starts managed workspace materialization with Notion child pages only. Database/data-source rows are intentionally out of scope because path, identity, and deletion semantics differ from normal child pages.\n\n## Desired outcome\n\nDefine a principled contract before adding implementation. The design should say whether database/data-source pages become folders, generated files, filtered views, or a separate command surface.\n\n## Acceptance criteria\n\n- Document the intended path mapping and stable identity model for database/data-source-backed pages.\n- Define conflict/deletion behavior separately from child-page materialization.\n- Add fake E2E coverage before live integration coverage.\n- Update the VRS spec once the behavior is chosen.

Start by reviewing PR #679 to understand the existing child-page workspace materialization and why database/data-source rows were excluded. Define the path, stable identity, conflict, and deletion contract, then add fake E2E coverage before live integration coverage. Update the VRS spec once the behavior is chosen.