…30170)

When a user sends a conversational follow-up while delegate_task is
running, gateway/run.py calls running_agent.interrupt(event.text) on
the PARENT agent. AIAgent.interrupt() then cascades synchronously
through self._active_children and calls interrupt() on every child
subagent, aborting in-flight delegate_task work. The user sees the
fallback cascade with no root-cause in the gateway log, and minutes of
subagent progress are destroyed — the exact failure mode reported in

Add GatewayRunner._agent_has_active_subagents(running_agent) — a
static helper that returns True iff the parent is currently driving
subagents via delegate_task. The helper is type-defensive: it ignores
truthy MagicMock auto-attributes (so this doesn't accidentally fire
in every test mock that hits the busy path), the _AGENT_PENDING_SENTINEL
placeholder, and missing locks.

Wire the helper into both interrupt branches:

  1. _handle_active_session_busy_message — the adapter-level busy
     handler. When busy_input_mode == 'interrupt' AND the parent has
     active subagents, demote to 'queue' semantics: skip the
     parent.interrupt() call, merge the message into the pending
     queue, and surface a dedicated ack ("⏳ Subagent working — your
     message is queued for when it finishes (use /stop to cancel
     everything).") so the operator knows the message wasn't lost and
     discovers the explicit escape hatch.

  2. The PRIORITY interrupt branch inside _handle_message — the
     non-command fast path. Same rationale, same demotion. Routes
     through _queue_or_replace_pending_event so the next-turn pickup
     stays unchanged.

Explicit /stop and /new commands take a completely different path
(_interrupt_and_clear_session in the slash-command dispatch at line
~6771) and are NOT affected by this guard — the operator still has a
way to force-cancel everything when they actually mean it. Configured
'queue' and 'steer' modes are also untouched: 'queue' already does the
right thing, and 'steer' goes through running_agent.steer() which does
NOT cascade to children (so subagents survive a steer too).

This is Phase 1 of the fix outlined in #30170 — the minimum viable
change that stops subagent loss. Phase 2 (delegation-aware steer
forwarding to active children) and Phase 3 (async delegation, #11508)
are intentionally out of scope.

Refs #30170.

The cherry-pick comment referenced 'line ~6771' for the /stop handler,
but on current main the handler is at a different offset. Remove the
hard-coded line number — the 'above' reference is sufficient.