Ship an automated code review step in the generated project's CI workflow

Part of #44. Came out of an ideation session, not a scoped request.

Add the chosen code review tool as a step in `templates/.github/workflows/ci.yml`, so a project scaffolded by `gonext init` gets automated PR review from day one without the developer setting anything up.

Open questions for whoever picks this up: whether the step is always present or gated behind a prompt at scaffold time, and how it behaves when the tool needs credentials the generated repository does not have yet.

Start with templates/.github/workflows/ci.yml and the scaffold path used by `gonext init`; read parent issue #44 for the ideation context. Define whether review is always included or prompt-gated, account for missing credentials, and verify that a generated project receives the intended automated PR-review workflow.

Written by the indexing model from the issue text.