Pin the runtime module version in the generated go.mod at scaffold time

Part of #37. Came out of an ideation session, not a scoped request.

Once the runtime is a module of its own, `gonext init` has to write a concrete version of it into the generated project's `go.mod` rather than letting `go mod tidy` resolve whatever is latest. Open question this issue should settle: which version a given CLI build pins, and what happens when the two tag lines drift.

Start by tracing the `gonext init` scaffolding path and where it writes the generated `go.mod`. Resolve which runtime version each CLI build should pin and define the behavior when the two tag lines drift; done means the issue has a settled versioning policy and the generated module uses that concrete version.

Written by the indexing model from the issue text.