A turn's hard interrupt fans out only to `_active_children`; background
delegate_task units are detached from the parent at dispatch
(`_dispatch_background` / honor_parent_interrupt=False), so /stop left them
running to completion and their result arrived minutes later as a parked
wake. Every stop surface now also calls
`tools.async_delegation.interrupt_for_session` for the session's units:
- gateway: `_interrupt_and_clear_session` (busy /stop, /new share it — /new
already did this in `_handle_reset_command`, the second request is
idempotent) and the idle `_handle_stop_command` tail, which replied
"No active task to stop." while a background child was running; it now
stops them and replies "Stopped".
- tui_gateway `session.interrupt` (Desktop Stop / TUI stop) — own UI sid +
spawner id only, so a viewer tab never kills gateway work.
- acp_adapter `cancel`.
- CLI `/stop` already used `interrupt_all` (process-wide); unchanged.
The stop recurses: depth>0 delegations are always synchronous
(`_model_background_value`), so the child's hard interrupt reaches its
workers through its own `_active_children` fan-out, and each level's
interrupted partial result rolls up as that child's completion.
An interrupted child's entry now carries what it actually had: the loop's
`final_response` is the "Operation interrupted." placeholder (also appended
as the closing assistant row), so `_build_result_entry` takes the child's
last real assistant text as `summary` and keeps the placeholder as `error`.
The unit finalizes normally and re-enters at once as its completion notice
(status=interrupted, "Partial output: ...", "Subagent Task Interrupted" on
TUI/Desktop) instead of the chat waiting for the child's budget to run out.
Docs: delegate_task description, tools/AGENTS.md, delegation.md,
gateway-session-lifecycle.md.
Part of #114456
b71359066c