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.

claude-plugins/plugins/base/skills/claude-subagents/SKILL.md

@@ -27,7 +27,7 @@ skill's recipe for you:
 start(name, prompt_file, model?, effort?, trigger?)
 continue(name, prompt, model?, effort?)
 status(name)
-interrupt(name, force?)
+interrupt(name)
 ```
 
 `start` and `continue` return once the turn is under way, not once it
@@ -42,7 +42,8 @@ reports how long since that turn last produced output, so a few seconds
 means it's working and an age climbing into the minutes means it's
 wedged and worth an `interrupt`. Reach for `continue` only once you
 actually have a new instruction for it, since that spends a turn.
-`interrupt` genuinely stops a running turn (`force: true` for SIGKILL).
+`interrupt` genuinely stops a running turn, with SIGINT so claude stops
+the commands it started; `status` says `stopping` until claude has exited.
 
 Model choice, prompt hygiene, splitting big batches, verify-then-report —
 everything else in this skill — applies exactly the same whether you're