feat: align local-first workspace direction

Document the VSCode-like local-first product shape, demote Convex to a control-plane role, and retire stale architecture drafts.

Add local workspace migration/export references plus smoke coverage for no-Convex managed workspace startup, local markdown title/body/options persistence, asset upload behavior, and Convex fixture export.

Verification: git diff --cached --check; node scripts/check-local-first-convex-guard.js --staged; node scripts/task444-convex-workspace-export-local-fixture-smoke.js; node scripts/task166-local-first-managed-workspace-no-convex-smoke.js; node scripts/task167-local-markdown-title-body-options-no-convex-smoke.js

package.json

@@ -9,6 +9,7 @@
     "dev:hot": "node scripts/dev-hot.js",
     "desktop:local": "node scripts/desktop-local.js",
     "desktop:electron": "node desktop-electron/dev-runner.js",
+    "check:local-first-convex-guard": "node scripts/check-local-first-convex-guard.js",
     "build:desktop:next": "node scripts/prepare-desktop-next.js",
     "dist:win": "pnpm run build:desktop:next && electron-builder --win nsis"
   },