chore: add CD workflow template and point spawn action to main (#19430)

## Summary

- Adds reusable composite GitHub Actions for Twenty app deployment:
- `.github/actions/deploy` — builds and deploys to a remote instance
(`api-url`, `api-key` inputs)
- `.github/actions/install` — installs/upgrades on a specific workspace
(`api-url`, `api-key` inputs)
- Adds a `cd.yml` CD workflow that calls both actions in sequence. The
workflow:
  - Deploys on push to `main`
  - Can be triggered from a PR by adding a `deploy` label
- Configures a named remote via `TWENTY_DEPLOY_URL` env var and
`TWENTY_DEPLOY_API_KEY` secret
- Applied to: `create-twenty-app` template, `postcard` example,
`hello-world` example
- Updates the `spawn-twenty-app-dev-test` action ref from
`@feature/sdk-config-file-source-of-truth` to `@main` in all `ci.yml`
files

packages/create-twenty-app/src/constants/template/github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
 
       - name: Spawn Twenty test instance
         id: twenty
-        uses: twentyhq/twenty/.github/actions/spawn-twenty-app-dev-test@feature/sdk-config-file-source-of-truth
+        uses: twentyhq/twenty/.github/actions/spawn-twenty-app-dev-test@main
         with:
           twenty-version: ${{ env.TWENTY_VERSION }}