notion-md: add managed workspace watch mode

Context\n\nPR #679 establishes managed workspaces via notion-md sync <page> <dir> and refreshes them with one-shot notion-md sync <dir>. The CLI intentionally rejects notion-md sync <managed-workspace> --watch for now because the existing watch loop only watches an already-resolved local file set.\n\n## Desired behavior\n\nA managed workspace watcher should periodically run workspace sync so newly discovered remote child pages are materialized, while preserving the existing guarded one-page sync behavior for files already present.\n\n## Acceptance criteria\n\n- notion-md sync <managed-workspace> --watch uses workspace metadata rather than recursive unmanaged file discovery.\n- Remote child pages created after watcher startup are materialized on a poll pass.\n- Local file events still coalesce into bounded sync passes.\n- Fake E2E coverage proves new remote child materialization during watch.

Start with PR #679 and the CLI paths for notion-md sync <page> <dir>, one-shot managed-workspace sync, and the existing --watch loop. Trace how workspace metadata and local file events are handled, then use the fake E2E coverage to verify that a remote child created after startup is materialized and local events still coalesce into bounded sync passes.