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

docs/web-ui/agent.md

@@ -227,11 +227,11 @@ Slash commands today:
 
 - `/help` — list commands locally.
 - `/clear` — wipe the local terminal view (server history kept).
-- `/cancel` — `POST /api/cancel` → host shellouts `pkill -INT
-  claude`, emits a Note. On an ACP agent it sends the agent
-  `session/cancel` instead, and kills it if the turn hasn't ended 10s
-  later. Also surfaces as a `■ cancel turn` button in the state row
-  while state=thinking.
+- `/cancel` — `POST /api/cancel` → SIGINT to the turn's claude and every
+  command it started (its process group), emits a Note. On an ACP agent
+  it sends the agent `session/cancel` instead, and kills it if the turn
+  hasn't ended 10s later. Also surfaces as a `■ cancel turn` button in
+  the state row while state=thinking.
 - `/compact` — `POST /api/compact` → sets the deferred
   `Bus::request_compact()` flag and returns immediately. The harness
   runs the `/compact` at the next turn boundary (end of the in-flight
@@ -272,8 +272,8 @@ shaped).
 - `POST /login/{start,code,cancel}` — claude OAuth login flow.
   Start/cancel emit `LiveEvent::StatusChanged` to flip the
   badge to/from `needs_login_in_progress`.
-- `POST /api/cancel` — SIGINT the in-flight claude turn. Emits a
-  `LiveEvent::Note`.
+- `POST /api/cancel` — SIGINT the in-flight claude turn and the
+  commands it started. Emits a `LiveEvent::Note`.
 - `POST /api/compact` — run `/compact` on the persistent session
   (same MCP config + system prompt + allowed tools as a normal
   turn — only the stdin payload differs). Flips state to