feat(delegation): structured stall metadata + live per-child status in /agents

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.

locales/zh-hant.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... 還有 {count} 個"
     running_processes:     "**執行中的背景程序：** {count}"
     async_jobs:            "**閘道非同步任務：** {count}"
+    background_delegations: "**背景委派：** {count}"
     none:                  "沒有作用中的代理或執行中的任務。"
     state_starting:        "啟動中"
     state_running:         "執行中"