Coordinator stuck "Working" while only a background subagent runs — queued input not answered or shown as pending

### Describe the bug

Sharing something I've hit several times in the CLI (v1.0.70). Upfront: I don't really know how Copilot's orchestration works internally — the log analysis below was done by Copilot itself at my request. The parts I can vouch for first-hand are what I saw in the UI.

• The busy indicator showed "Working", and prompts I typed were pushed to a queue — not answered, and not shown as "(pending)".
• I tried Ctrl+X B (move task to background) and it said "No running sync task to move to background" — so the CLI itself says nothing sync is running, yet it's "Working" and queueing my input.
• In /tasks , the only thing running was one subagent, marked [background] .
• The coordinator became responsive only after I manually cancelled that background subagent in /tasks . Cancelling the sole [background] task is what released my queued prompt — which makes it look like a "background" task was actually blocking the main agent.

### Affected version

v1.0.70

### Steps to reproduce the behavior

_No response_

### Expected behavior

if the coordinator is free it should answer immediately; if busy, it should take my prompt as steering and show "(pending)" and act next turn. Actual: queued for minutes with nothing visibly happening except a background subagent.

### Additional context

From Copilot's log analysis (may be imperfect, not internal state): it found two windows where the coordinator went quiet for ~13–17 min with only a background subagent running. In the clearest one, the subagent-completion notification actually fired fine — so this isn't just a "dropped notification"; it looks more like the coordinator being blocked by / kept "Working" behind its own background subagent, and/or how typed input is classified while a subagent runs. It also ruled out sync-blocking (all read_agent calls were wait:false , tasks were mode:background ).

首先在 CLI v1.0.70 中重現該行為，同時觀察 /tasks、Working 指示器、排入佇列的輸入以及 Ctrl+X B。比較只有一個 [background] 子代理的工作階段，並檢查報告中所述的 read_agent 日誌視窗；當 coordinator 回應，或在不需要手動取消的情況下將輸入明確標記為待處理時，即表示完成。

由索引模型根據 Issue 內容生成。