Publish a background-worker port (next after auth)

Moved off `roadmap.md`'s *Published Contract Candidates* table, where the verdict was **Next after auth**.

The *Scaffolding CLI Design Concept* already offers River, Machinery and Watermill as a scaffold-time engine choice. Three implementations of one concept is the definition of a port, and a third party writing a Temporal or SQS adapter is entirely plausible — so it clears both clauses of the bar in `roadmap.md`: at least two plausible implementations, one of which someone outside this repo might write.

Highest-value candidate once a worker feature pack exists.

**Sequencing rule applies:** publish only once the *second* implementation actually exists. A port with one implementation is a guess dressed as an abstraction, and a published contract cannot be withdrawn without breaking consumers. Blocked on the background-worker pack landing with at least two engines.

Read roadmap.md and the Scaffolding CLI Design Concept, then inspect the background-worker feature pack as it develops. Confirm that a second worker engine exists before defining the port; done means the published contract supports the demonstrated implementations and remains open to a third-party Temporal or SQS adapter.

Written by the indexing model from the issue text.