feat: auto-prune inactive terminal sessions

[zzj3720]

Summary

• add a worker-side SessionJanitor that sweeps inactive terminal sessions on startup and on an interval
• delete persisted session, inbound, and background-job state plus workspace/job/log artifacts when a session is eligible
• add config knobs, docs, and coverage for the janitor path

Testing

• pnpm test
• pnpm build
• manual smoke: built SessionJanitor against temp broker state and verified the stale session record and workspace directory were removed