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

hive-subagent-mcp/Cargo.toml

@@ -15,6 +15,7 @@ hive-agent-sock.workspace = true
 hive-claude.workspace = true
 hive-sock-client.workspace = true
 hive-types.workspace = true
+libc.workspace = true
 rmcp.workspace = true
 schemars.workspace = true
 serde.workspace = true