subagent daemon: report a killed session as killed, not idle

A subagent whose claude process died on a signal — the kernel's OOM
killer, a stopped unit, an `interrupt` — was indistinguishable from one
that finished its turn: its entry left the `running` map, `status` fell
through to "a session exists on disk" and answered `idle`, and the
end-of-turn todo said the subagent had "finished". The usual next move
on that reading is `continue`, which resumes work that was cut mid-turn
with nothing having recorded that it was cut.

The driver already preserves how the child ended — `RunningClaude::wait`
returns `Error::Exit` carrying the `ExitStatus`, whose `signal()` is the
whole answer — so this reads it rather than having to recover it:
`classify_end` turns the outcome into `Complete` / `Killed { signal }` /
`Failed`, and `State::finish_turn` remembers a kill against the name
(cleared by the next confirmed spawn under it).

What an agent sees as a result:

- `status` reports the session killed, naming the signal, instead of idle;
- the todo the daemon pushes without being asked says the subagent was
  KILLED mid-turn rather than that it finished;
- `continue` still resumes such a session, but its reply says the
  previous turn was killed, so no caller carries on from cut-off work
  believing it was complete.

Refs #4326

docs/tools/subagent.md

@@ -33,6 +33,26 @@ claude's own on-disk session (`hive_claude::SessionStore`), which
 `continue` reattaches to independent of the daemon's own lifetime — a
 restart loses the _in-flight turn_, not the subagent's history.
 
+## A killed turn
+
+A subagent whose `claude` process dies on a signal — the kernel's OOM
+killer under memory pressure, a stopped unit, an `interrupt` from the
+owning agent — did not finish its turn, and the daemon says so rather
+than letting it settle back into `idle`:
+
+- `status` reports the session **killed**, naming the signal, instead of
+  the `idle` it reports for a turn that ended on its own;
+- the end-of-turn todo the daemon pushes without being asked says the
+  subagent was killed mid-turn, not that it finished;
+- `continue` still resumes such a session — often what you want — but
+  its reply says the previous turn was killed, so nobody carries on from
+  cut-off work believing it was complete.
+
+The record is per-name, in memory with the rest of this daemon's state,
+and the next confirmed spawn under that name clears it. A daemon restart
+loses it along with everything else — the daemon has to outlive the kill
+to report it, which is what `OOMPolicy=continue` on the unit is for.
+
 ## Compaction trade-off
 
 Built on `hive_claude::Claude::spawn` + `RunningClaude::wait` directly