[Feature Request] Main prompt remains interactive while agents work in background

Would it be possible for the main Claude session to stay responsive while agents execute in background?

Current behavior: When Claude delegates to Task agents, the prompt is blocked until the agent returns. For long-running tasks this means minutes of no interactivity.

Proposed behavior:

• Claude delegates substantive work via run_in_background: true by default
• Main prompt returns immediately — user can ask questions, give new tasks, check status
• Agent results are reported at the next natural break
• Single terminal, single session — no multi-pane UI needed

This differs from #22034 (agent frontmatter flag) and #26376 (task queue) in that the ask is for the main session to never be blocked, not just for specific agents to run in background.

The input-during-execution bugs (#20431, #26388) are symptoms of this — user input during agent work is currently lost or misinterpreted because the session isn't designed to accept it.