Support a host-prefixed Go module path at init

`gonext init` names the module after the slug alone, so a generated project declares `module golden-app` rather than `module github.com/org/repo`. That is fine until the project is pushed somewhere and anything wants to import it, at which point the module has to be renamed by hand across every file. The init CLI design deferred this twice — once in *Explicitly deferred (not phase 1)* and again in *Open items carried to later phases* (`docs/superpowers/specs/2026-08-27-gonext-init-cli-design.md`).

Open: whether the host prefix is a flag, a second prompt, or inferred from the git remote when one exists, and how it interacts with the `[PROJECT-NAME]` substitution token.

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

Start with docs/superpowers/specs/2026-08-27-gonext-init-cli-design.md, especially “Explicitly deferred (not phase 1)” and “Open items carried to later phases.” Trace the existing `gonext init` flow and its `[PROJECT-NAME]` substitution, then document and implement the chosen host-prefix behavior, including the expected generated module path.

Written by the indexing model from the issue text.