claude runtime: interrupting a session kills every process it started

hive-claude 0.1.3 spawns claude as the leader of its own process group,
and its `Cancel::cancel` and idle watchdog signal that group instead of
claude's pid alone. Commands claude started (a `nix build`, a test run)
used to survive a subagent `interrupt` and keep running.

Bump hive-claude to 0.1.3. The subagent `interrupt` call site needs no
code change: `Cancel::cancel(force)` kept its signature.

The harness's `/cancel` and `/logout` did not go through hive-claude:
they ran `kill -INT <pid>` on the claude child they found in /proc. They
now SIGINT the group that child leads, through `libc::kill`. A `claude`
child that leads no group (the `claude plugin` calls) shares the
harness's group and is still signalled alone.

Tests: an interrupted subagent's grandchild dies, for both SIGINT and
`force`, and fails on hive-claude 0.1.2; a harness cancel stops a
turn's grandchild, and fails when only the pid is signalled.

docs/tools/subagent.md

@@ -166,6 +166,11 @@ from how the child exited. `continue` clears it and hands the session a
 fresh turn allowance, which is what makes an interrupt a pause you can
 undo rather than a session you have to abandon.
 
+On `claude`, the signal goes to claude's whole process group, so the
+commands the subagent started (a `nix build`, a test run) get it too. The
+signal doesn't reach a command that moved itself out of the group
+(`setsid`).
+
 When the session knows where its report goes — `start`'s `report_file`,
 or the path the subagent names when it signals — the daemon appends the
 stop reason to that file as well, so the artifact you were going to read