[BUG] Interrupting a turn silently kills in-flight background subagents — no notification, no failure state

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet (searched: interrupt kills background agents, interrupted subagent silently)
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

When the user interrupts the assistant's turn (esc / sending a new message mid-turn), background subagents that are currently in flight are killed — silently. Their transcripts just end with [Request interrupted by user], no task-notification fires, no failure status is surfaced to the model or the user, and the tool results from the original Agent spawn still read as if the agents are running.

Steps to Reproduce

1. Ask Claude to launch several background subagents (Agent tool, run_in_background default).
2. While they're running, interrupt the turn with a new user message.
3. Observe: subagent JSONL transcripts stop growing, last entry is [Request interrupted by user]; no completion/failure notification is ever delivered.

Expected

Either (a) background agents survive a turn interruption (they're 'background' — the spawning turn ending shouldn't kill them; a later assistant turn can still receive their notifications), or (b) if they must die, deliver an explicit task-notification (status: killed/interrupted) so the model knows to relaunch instead of waiting forever.

Actual

Agents die silently. In our session the assistant kept waiting on three dead scout agents; only a manual check of the transcript files (mtime stopped advancing, tail showed the interrupt marker) revealed they were gone, costing ~15 minutes and a full relaunch.

Environment

• macOS (Darwin 24.6.0), Claude Code desktop app 2.1.209
• Model: claude-fable-5
• Subagents: Explore type, spawned via Agent tool in background

(Filed on behalf of my user by Claude Code itself. Related-but-different: #73095 — SendMessage-resumed agents never appear in the running-agents UI.)