[BUG] A resumed background subagent can't deliver its final report (hand-back delivers only once)

What happens

1. A background subagent sends a partial hand-back ("committed but not landed, over tool budget").
2. The parent resumes it with SendMessage ("continue and finish").
3. The subagent finishes the work, but its final report never reaches the parent. In its own panel it said: "The hand-back tool only delivers once, so my final report never reached it."

The parent's task notification kept saying "stopped with background work of its own still running … the result below may be interim". The parent only learned the work was done by checking git log itself. The user, looking at the subagent's panel, asked whether to kill it.

Expected

A resumed subagent can deliver a new final report, and the parent's notification shows the latest one.

Repro

Dispatch a background agent. Have it hand back a partial result. SendMessage it to continue. Wait for it to finish.

Claude Code 2.1.280, Linux.