feat: make each plugin an npm workspace built by a termix-plugin CLI

plugins/workspaces/README.md

@@ -0,0 +1,22 @@
+# Workspaces
+
+Save and restore named tab/split layouts per user.
+
+Bundled with Termix and enabled by default.
+
+## Layout
+
+    manifest.json        id, capabilities and what this plugin contributes
+    src/backend/         activate(ctx) / deactivate(), plus the routes
+    src/frontend/        the UI it registers
+    locales/en.json      its strings (English only; the rest are translated)
+    tests/backend/       vitest, node
+    tests/frontend/      vitest, jsdom
+
+## Commands
+
+    npm run build     bundle into dist/
+    npm run test      run this plugin's tests
+    npm run typecheck
+
+See packages/plugin-sdk/ARCHITECTURE.md for the contract this follows.