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

infra/convex/README.md

@@ -2,6 +2,8 @@
 
 本目录用于在本机通过 Docker Compose 启动 Convex backend + dashboard，并使用 backend 自身的持久化卷存储（包含数据库与文件存储）。
 
+在 local-first 方案下，这套 Convex 自托管环境定位为控制面、cloud source、compat 和可选 sync replica 运行底座；页面正文、默认附件和本地 AI 会话全文不再以它作为早期产品默认数据真相。
+
 ## 启动
 
 1. 确保仓库根目录 `.env.all` 存在（全局唯一 env 文件）。