Gap: open plugin system for third-party feature packs

**Seen in:** Buffalo (`buffalo generate` plugins) · **Priority:** Low–Medium

Feature Packs A–I (`README.md`) are a fixed, curated list. There is no path for a third party to ship their own installable pack/generator.

**Deferred** until the CLI and the existing packs are stable.

*Published Contract Candidates* in `roadmap.md` is the groundwork: each published port is one plugin point, and that section's `httpx` note records what still blocks a pack from shipping as an installable module rather than generated code.

**Narrowed since this was first written:** the auth port now *is* a published contract (`github.com/dennys-bd/gonext/auth`), so a third-party identity provider can already ship as an installable module compiled against a stable type — the first real plugin point in the tree.

Start with README.md to understand the fixed Feature Packs A–I and then read the Published Contract Candidates section of roadmap.md. Review the existing auth contract at github.com/dennys-bd/gonext/auth and the httpx note, plus the Buffalo buffalo generate plugin model mentioned in the issue. Done would mean a defined path for third parties to ship an installable pack or generator, but the scope is currently deferred and not specified.

Written by the indexing model from the issue text.