Background agent completion notifications delayed until user interrupts

Description

When using the Agent tool with run_in_background: true, completion notifications are not delivered in real-time. Instead, they are buffered and only appear when the user presses Escape, sends a message, or otherwise interrupts the conversation.

Steps to Reproduce

1. Launch 4 background agents simultaneously using the Agent tool
2. Continue working on other tasks while agents run
3. Agents complete at different times (verified by checking output file sizes which stop growing)
4. No completion notifications appear
5. Press Escape → all 4 completion notifications arrive simultaneously

Expected Behavior

Background agent completion notifications should be delivered as soon as the agent completes, interrupting the current conversation flow if needed (similar to how terminal command completions work).

Actual Behavior

Notifications are queued and only delivered when the user takes an action (Escape, new message, etc.). This defeats the purpose of background agents for parallel work — the user has no way to know when agents finish without manually checking.

Environment

• Claude Code CLI (latest)
• macOS Darwin 25.2.0
• Model: claude-opus-4-6[1m]

Impact

High — background agents are a key parallelism feature but without timely notifications, users don't know when results are ready. This session had 4 investigation agents that completed but results weren't available until ~10 minutes later when the user happened to press Escape.