subagents, ACP: stop with SIGINT only, so claude ends its own tool commands

claude's Bash tool starts every command in a new session, so a signal to
claude's process group never reaches it. claude stops those commands
itself when SIGINT makes it shut down. A SIGKILL leaves them running,
re-parented to init. Per the ruling on #4999, every stop is SIGINT and
nothing escalates.

- Subagent `interrupt` always sends SIGINT. The `force` argument is gone
  from the MCP tool and from `session::interrupt`. A caller that still
  sends `force: true` gets SIGINT; the argument is ignored.
- `interrupt` used to free the session name before it signalled, so a
  `continue` could start a second claude on the session while the first
  was still exiting. The `running` map now holds a three-state `Slot`
  (starting, running, stopping). An interrupted claude turn is
  `Stopping` until the turn task's own `finish_turn` clears it. `status`
  says "stopping", and `start`/`continue` on that name are refused.
  `interrupt` still returns at once. The ACP arm is unchanged: its name
  is freed when `session/cancel` is sent.
- ACP `Connection` drop sends its process group SIGINT, not SIGKILL, and
  the spawn no longer sets tokio's `kill_on_drop`, which SIGKILLed the
  leader after the `Drop` body. The test fixture's background grandchild
  resets SIGINT to default, since `&` starts it ignored.

Tests: a fake claude that records SIGINT, interrupted, fails with
`cancel(true)` (the old `force: true` path). An interrupted fake claude
that keeps running keeps its name claimed and `continue` is refused,
which fails when the slot is freed before the signal. A fake ACP agent
that records SIGINT, dropped, fails with SIGKILL in `Drop`, and also with
SIGINT in `Drop` but `kill_on_drop(true)` left in place.

Not changed: hive-claude's idle watchdog (still SIGKILL, separate
hive-claude change), the units' `KillSignal`, `machinectl kill`, and the
login helper.

hive-runtime/README.md

@@ -35,13 +35,15 @@ The crate depends on no hyperhive binary crate, so `hive-agent` and
 A turn is stopped with `session/cancel`: by the `Canceller` handle, or by the
 idle watchdog once no `session/update` has arrived for
 `Config::idle_timeout`. An agent that has not answered the prompt 10s later
-is killed, and the next turn respawns it. A watchdog stop fails the turn
-with `AcpError::IdleTimeout` (`IdleKilled` if the agent was killed). This is
+is sent SIGINT, and the next turn respawns it. A watchdog stop fails the turn
+with `AcpError::IdleTimeout` (`IdleKilled` if the agent was sent SIGINT). This is
 the only way a provider error the agent retries without reporting it, such as
 an HTTP 429, ends the turn.
 
-The agent leads its own process group. Killing it, and dropping the runtime,
-`SIGKILL`s that group, so the commands the agent started die with it.
+The agent leads its own process group. Stopping it, and dropping the runtime,
+sends that group SIGINT, so the commands the agent started get it too.
+Nothing escalates to SIGKILL: an agent that ignores SIGINT keeps running,
+and the next turn starts a new one beside it.
 
 ## ACP backend: compaction