docs: scope out the openQA worker protocol

[mimi1vx]

Why

ruoqa is an external REST client. Three routes are reachable only from
inside a running job on an os-autoinst worker:

• multipart artefact upload (POST /api/v1/jobs/:id/artefact) — the only
  file-upload route in the whole v1 API, and it 404s with No worker assigned unless a worker is currently running the job
• its chunked-asset variant
• the X-API-JobToken routes (whoami, mutex, barrier, mm) that
  os-autoinst test modules use to synchronise during a job run
• the worker WebSocket (/api/v1/ws/:workerid)

Implementing any of this means implementing a worker — a different program
with a different auth model (JOBTOKEN), lifecycle, and wire format, not a
gap in this REST client.

What changed

• README.md's ## Behaviour section states the exclusion next to the
  existing scope bullets (no typed response models, no CLI binary) and
  names the four excluded surfaces.
• Documents the escape hatch that already exists for a caller who does hold
  a job token: PreparedRequest's fields are public and Client::execute
  takes one directly, backed by a no_run doctest so it can't silently
  drift.

Documentation-only change — no src/ files touched.

Verification

• cargo fmt --check
• cargo clippy --all-targets --locked -- -D warnings
• cargo test --locked (doctest compiles)
• cargo check --locked --all-targets
• cargo deny check