feat: rebuild workspaces as the reference plugin

plugins/workspaces/README.md

@@ -2,12 +2,15 @@
 
 Save and restore named tab/split layouts per user.
 
-Bundled with Termix and enabled by default.
+Bundled with Termix and enabled by default. This is the reference
+conversion: it imports only `@termix/plugin-sdk`, and "Converting a feature
+into a plugin" in packages/plugin-sdk/ARCHITECTURE.md points here for each step.
 
 ## Layout
 
     manifest.json        id, capabilities and what this plugin contributes
-    src/backend/         activate(ctx) / deactivate(), plus the routes
+    src/backend/         activate(ctx), the table, repository and routes
+    migrations/          one .sql per dialect, generated from src/backend/tables.ts
     src/frontend/        the UI it registers
     locales/en.json      its strings (English only; the rest are translated)
     tests/backend/       vitest, node
@@ -18,5 +21,6 @@ Bundled with Termix and enabled by default.
     npm run build     bundle into dist/
     npm run test      run this plugin's tests
     npm run typecheck
+    npx termix-plugin migrations <name>   after changing src/backend/tables.ts
 
 See packages/plugin-sdk/ARCHITECTURE.md for the contract this follows.