Add delegation, handoff, and background-session support #50

Problem

The control plane must support agents handing work to each other and continuing long-running work while the operator uses other parts of the app.

Scope

• Track delegation, handoff, reviewer-worker, and planner-executor style flows
• Cover background sessions and queued jobs that continue outside the foreground session view
• Keep the operator able to inspect and re-enter the work later

Out of scope

• Fully distributed remote-worker deployment
• Detailed workflow-engine internals outside the product expectations

Implementation notes

• Make delegation and background work explicit product features, not hidden runtime behavior
• Align the issue with the session composer and fleet board
• Support both coding and non-coding agent patterns

Definition of Done

• The issue defines how delegation and background work should appear in the product
• Later implementation can proceed without re-deciding the operator model

Verification

• Review the issue against the feature spec and runtime orchestration epic

Dependencies

• Parent epic: [Epic] Build the agent catalog and fleet-management experience #16
• Related epic: [Epic] Add the embedded agent runtime host to the Uno desktop app #12