Design the feature-pack gating mechanism in scaffold.Copy()

The template tree ships every file unconditionally; nothing in `Copy()` can include or omit a file based on what the developer selected. The init CLI design named this phase 2 and left it undesigned, sketching directory- or manifest-based inclusion for pack-owned files plus conditionals on a handful of genuinely shared ones — `go.mod`, `docker-compose.yml`, the wire provider list, `.env.example` (`docs/superpowers/specs/2026-08-27-gonext-init-cli-design.md`, *Open items*).

Worth registering now because #13 introduces a path-ownership map for selecting agent tooling, which is the same problem at smaller scale. Either that map generalises into the pack mechanism or the two solve the same thing twice — better to decide deliberately than to discover the duplication later. `gonext add ` (#15) depends on whatever this settles.

Came out of reviewing the deferred sections of the existing specs.

Read the Open items section of docs/superpowers/specs/2026-08-27-gonext-init-cli-design.md, then inspect scaffold.Copy() and the path-ownership work in #13. Compare that with the dependency from #15 and document a decided gating approach covering pack-owned files and the listed shared files, including how it avoids duplicate ownership logic.

Written by the indexing model from the issue text.