Design: headless/queue-driven mode

[cyberstorm-builder]

Summary

• add design document for headless / queue-driven execution (docs/plans/2026-03-12-headless-queue-design.md)
• outline workflows, job schema, CLI entrypoint, Redis consumer, and result publishing
• capture open questions on Redis flavor, workflow set, sandbox expectations, retention/ack policy, and auth

Testing

• n/a (docs only)

[cyberstorm-reviewer]

Targets main; policy requires PRs to target dev. Please retarget to dev. Also missing required issue link (Closes #) per review standards; add the correct issue reference.