[starter] Add a fixture-only bounded job adapter example

[ashishki]

Problem

The contributor boundary permits bounded adapters, but a new contributor needs one small reference implementation that demonstrates the contract without a provider or external service.

Bounded scope

Add a fixture-only adapter example covering versioned input/output, redaction, timeout, retry classification, deterministic cost, and artifact generation.

Acceptance criteria

• The example runs through the existing local worker path.
• Tests cover success, retryable failure, terminal failure, timeout, cancellation, and redacted evidence.
• Model or adapter output cannot choose retry budget, terminal state policy, or evidence integrity.
• Documentation maps each field to docs/AUTONOMOUS_ROUTINE_CONTRACT.md and includes an exact local command.
• No provider SDK, credentials, network endpoint, user data, or production claim is added.

Non-goals

No plugin marketplace, arbitrary-code sandbox, hosted control plane, or independently owned workflow claim.