sdk: a Ruby client over the producer door

Ruby has a large background-job population and Sidekiq is the incumbent. A Ruby client over the
producer door is a self-contained contribution that needs no Rust and no build matrix entry —
exactly the shape the remote tier was built to enable.

Scope: enqueue, batch enqueue, job read, cancel, queue counts. Producer side only; executing
tasks is the executor door and out of scope.

Prerequisites, so this is not started against a moving target:

• REMOTE_SDK_CONTRACT.md (#828) defines the surface and the encoding rules
• contracts/wire-vectors.json is the conformance bar — a Ruby CBOR encoder must emit
  definite-length containers and shortest-form integers, and the vectors are how you know

Good first issue for someone who writes Ruby. It does not require knowing this codebase, only
the contract.

Start with REMOTE_SDK_CONTRACT.md (#828) to identify the producer-client surface and encoding rules, then inspect contracts/wire-vectors.json as the conformance reference. Cover enqueue, batch enqueue, job read, cancel, and queue counts on the producer side only; done means the Ruby client matches the contract and emits the required CBOR forms.