`gonext add <pack>` — retrofit a feature pack onto a generated project

Adds a feature pack (Redis caching, background worker, transactional email, …) to a project that was generated without it — e.g. a project that started API-only and now needs a queue.

**Blocked by #64.** Today `gonext init` has no pack selection at all (only `--agents`), so there is nothing to retrofit yet. This is the general mechanism; `gonext add agent` is the first concrete subcommand and lands independently. Once #64 defines how packs are gated in `scaffold.Copy()`, this becomes the same selective copy applied to an existing tree.

Split out of #15.

Start by reading the resolution of #64 and the pack-gating behavior in scaffold.Copy(). Then trace how an existing generated project would be handled by the CLI. Done means the general selective-copy mechanism can retrofit a chosen feature pack onto an existing project tree, beyond the independently scoped `gonext add agent` command.

Written by the indexing model from the issue text.