Give a signalled agent the exit code a shell would

[wilbeibi]

#13 passed the launched agent's exit code through, but a signal carries no exit code, so one ending was left behind:

$ catchup fork claude          # the agent is OOM-killed
catchup: signal: killed
$ echo $?
1

That is catchup's name over a death that was not catchup's — the attribution #13 removed everywhere else — and a wrapper learns nothing from the 1. It is also the shape #12's triage rule misreads: an agent told that "a non-zero exit from fork is the launched agent's own status" sees a catchup:-prefixed error here and files a bug.

A signalled agent now exits 128 plus the signal number, which is the answer a shell gives for the same death, so $? is 137 for a kill -9 and 139 for a segfault. The death still gets one line, because the agent had no chance to explain itself and the kernel says nothing to the terminal — but the line names the agent:

catchup: claude died: killed
catchup: claude died: terminated
catchup: claude died: segmentation fault

syscall.WaitStatus.Signaled() and .Signal() are defined on Windows and macOS as well as Linux, so this needs no build tags; on Windows a terminated process still carries an exit code and takes the existing path. Verified end to end with SIGKILL, SIGTERM and SIGSEGV (137/143/139); the new test skips on Windows.