fix(generators): emit cms barrel in feature template

The feature generator emitted no `src/integrations/cms/index.ts`, but
`gen job` and `gen event consume` both `assertAnchors` on that file's
`<gen:job-tasks>` anchor — so they failed on any freshly-scaffolded
feature. Add a `cms/index.ts.hbs` template carrying the anchor, wire the
add action into the generator, and export `./cms` from the package.json
template to match all five existing features.

turbo/generators/templates/feature/package.json.hbs

@@ -6,6 +6,7 @@
   "exports": {
     ".": "./src/index.ts",
     "./ui": "./src/ui/index.ts",
+    "./cms": "./src/integrations/cms/index.ts",
     "./api": "./src/integrations/api/router.ts",
     "./di/bind-production": "./src/di/bind-production.ts",
     "./di/bind-dev-seed": "./src/di/bind-dev-seed.ts"