Adds /notion-sync per the conditions agreed in discussion #166: tool-agnostic sync contract with Notion as the in-tree reference binding, silently optional (covers unconfigured, headless, and unauthenticated states), read-only toward the repo with the gitignored sync-state file as its only local write, write-once page bodies, documents sync as filenames only. Complements /html-report: deep local dashboard vs glanceable anywhere-view.
