feat(subagent): let ancestors interrupt descendants

interrupt_agent(agent_id) passes the calling agent as the ancestor
authority for ctx.subagents.interrupt(); the service verifies live
registry identity and recorded lineage, so a direct child or deeper
descendant stops with the same generic parameter while send_message keeps
its exact-direct-parent authority.

Discovery: list_agents gains an optional scope. descendants walks the new
SubagentService.listDescendants() — one lineage trace flattened in stable
pre-order across ordinary and one-shot intermediates, each entry carrying
its verified parentId and depth — and every status now comes from the
live Agent registry (running/idle/complete).

Refs #1535

apps/web/tests/shipped-composition.e2e.ts

@@ -30,6 +30,7 @@ const EXPECTED_TOOLS = [
   'edit',
   'exit_plan_mode',
   'get_goal',
+  'interrupt_agent',
   'list_agents',
   'ralph',
   'read',