Completes #51690 on top of the salvaged #60378 timeout metadata:
- async_delegation: terminal 'stalled' events now carry structured
stall context (stalled_after_quiet_seconds, stall_threshold_seconds,
stall_phase idle|in_tool, stall_grace_seconds) on both single and
batch paths, persisted in the durable row so restart-restored events
keep it. Mirrors the sync path's timeout_seconds/timed_out_after_
seconds/timeout_phase from #60378.
- list_async_delegations(): exposes seconds_since_progress and live
children_activity (per-child api_calls, current_tool,
seconds_since_activity) sampled from the dispatch's progress_fn
outside the records lock; private monitor bookkeeping and callables
never leak.
- /agents (CLI + gateway): background delegations render per-child
activity rows, quiet-time hints, and the stalling state; gateway
section is new (previously async delegations were invisible there).
New locale key gateway.agents.background_delegations in all 17
catalogs.
Tests: stall-metadata event shape, live-listing projection, gateway
/agents rendering (real registry dispatch, sabotage-verified), sync
timeout metadata fields, non-timeout None contract.
