`gonext generate` — refresh or check the Wire DI output

Bare `gonext generate` regenerates `wire_gen.go` from the DI providers; a check mode (e.g. `--check`) verifies it is not stale, the same check CI and the pre-commit hook already enforce. Thin wrapper around the existing `make generate` / `make generate-check` targets — the CLI adds nothing here beyond a single entry point, so it must not duplicate that logic.

Split out of #15.

Start with the existing make generate and make generate-check targets, then inspect how the CLI commands are registered. Use wire_gen.go and the existing CI and pre-commit checks to confirm the expected generation and stale-output behavior. Done means bare gonext generate refreshes the output and a check mode verifies it without duplicating the make-target logic.

Written by the indexing model from the issue text.