`gonext generate worker <name>` — background job skeleton (pack-gated)

Scaffolds a new job skeleton for whichever background-worker engine the project selected (River / Machinery / Watermill). Only available when a background-worker pack is present in the project; otherwise the command explains what to add.

**Blocked by #64** (no pack selection exists yet) and by a background-worker pack actually shipping. #32 (published background-worker port) is related but is the library contract, not the pack.

Split out of #15.

Start with blocked issue #64 to understand pack selection, then check whether a background-worker pack has shipped. The command is done when `gonext generate worker ` scaffolds a job for the selected River, Machinery, or Watermill engine and explains what to add when no background-worker pack is present.

Written by the indexing model from the issue text.