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/af.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... en nog {count}"
     running_processes:     "**Lopende agtergrondprosesse:** {count}"
     async_jobs:            "**Asinchrone werke van die gateway:** {count}"
+    background_delegations: "**Agtergrond-delegasies:** {count}"
     none:                  "Geen aktiewe agente of lopende take nie."
     state_starting:        "begin"
     state_running:         "loop"

locales/ar.yaml

@@ -58,6 +58,7 @@ gateway:
     more:                  "... و{count} آخر"
     running_processes:     "**العمليات الخلفية الجارية:** {count}"
     async_jobs:            "**مهام البوابة غير المتزامنة:** {count}"
+    background_delegations: "**التفويضات في الخلفية:** {count}"
     none:                  "لا يوجد وكلاء نشطون أو مهام جارية."
     state_starting:        "قيد البدء"
     state_running:         "قيد التشغيل"

locales/de.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... und {count} weitere"
     running_processes:     "**Laufende Hintergrundprozesse:** {count}"
     async_jobs:            "**Gateway-Async-Jobs:** {count}"
+    background_delegations: "**Hintergrund-Delegationen:** {count}"
     none:                  "Keine aktiven Agenten oder laufenden Aufgaben."
     state_starting:        "startet"
     state_running:         "läuft"

locales/en.yaml

@@ -68,6 +68,7 @@ gateway:
     more:                  "... and {count} more"
     running_processes:     "**Running background processes:** {count}"
     async_jobs:            "**Gateway async jobs:** {count}"
+    background_delegations: "**Background delegations:** {count}"
     none:                  "No active agents or running tasks."
     state_starting:        "starting"
     state_running:         "running"

locales/es.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... y {count} más"
     running_processes:     "**Procesos en segundo plano en ejecución:** {count}"
     async_jobs:            "**Tareas asíncronas del gateway:** {count}"
+    background_delegations: "**Delegaciones en segundo plano:** {count}"
     none:                  "No hay agentes activos ni tareas en ejecución."
     state_starting:        "iniciando"
     state_running:         "en ejecución"

locales/fr.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... et {count} de plus"
     running_processes:     "**Processus d'arrière-plan en cours :** {count}"
     async_jobs:            "**Tâches asynchrones du gateway :** {count}"
+    background_delegations: "**Délégations en arrière-plan :** {count}"
     none:                  "Aucun agent actif ni tâche en cours."
     state_starting:        "démarrage"
     state_running:         "en cours"

locales/ga.yaml

@@ -57,6 +57,7 @@ gateway:
     more:                  "... agus {count} eile"
     running_processes:     "**Próisis chúlra ag rith:** {count}"
     async_jobs:            "**Tascanna asincrónacha gateway:** {count}"
+    background_delegations: "**Tarmligin sa chúlra:** {count}"
     none:                  "Níl aon ghníomhairí gníomhacha ná tascanna ag rith."
     state_starting:        "ag tosú"
     state_running:         "ag rith"

locales/hu.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... és még {count}"
     running_processes:     "**Futó háttérfolyamatok:** {count}"
     async_jobs:            "**Átjáró aszinkron feladatai:** {count}"
+    background_delegations: "**Háttérdelegálások:** {count}"
     none:                  "Nincsenek aktív ügynökök vagy futó feladatok."
     state_starting:        "indul"
     state_running:         "fut"

locales/it.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... e {count} altri"
     running_processes:     "**Processi in background in esecuzione:** {count}"
     async_jobs:            "**Job asincroni del gateway:** {count}"
+    background_delegations: "**Delegazioni in background:** {count}"
     none:                  "Nessun agente attivo o attività in esecuzione."
     state_starting:        "in avvio"
     state_running:         "in esecuzione"

locales/ja.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... 他に {count} 件"
     running_processes:     "**実行中のバックグラウンドプロセス:** {count}"
     async_jobs:            "**ゲートウェイ非同期ジョブ:** {count}"
+    background_delegations: "**バックグラウンド委任:** {count}"
     none:                  "アクティブなエージェントや実行中のタスクはありません。"
     state_starting:        "起動中"
     state_running:         "実行中"

locales/ko.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... 외 {count}개 더"
     running_processes:     "**실행 중인 백그라운드 프로세스:** {count}"
     async_jobs:            "**게이트웨이 비동기 작업:** {count}"
+    background_delegations: "**백그라운드 위임:** {count}"
     none:                  "활성 에이전트나 실행 중인 작업이 없습니다."
     state_starting:        "시작 중"
     state_running:         "실행 중"

locales/pt.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... e mais {count}"
     running_processes:     "**Processos em segundo plano em execução:** {count}"
     async_jobs:            "**Tarefas assíncronas do gateway:** {count}"
+    background_delegations: "**Delegações em segundo plano:** {count}"
     none:                  "Não há agentes ativos nem tarefas em execução."
     state_starting:        "a iniciar"
     state_running:         "em execução"

locales/ru.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... и ещё {count}"
     running_processes:     "**Выполняющиеся фоновые процессы:** {count}"
     async_jobs:            "**Асинхронные задачи шлюза:** {count}"
+    background_delegations: "**Фоновые делегирования:** {count}"
     none:                  "Нет активных агентов или выполняющихся задач."
     state_starting:        "запускается"
     state_running:         "выполняется"

locales/tr.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... ve {count} tane daha"
     running_processes:     "**Çalışan arka plan süreçleri:** {count}"
     async_jobs:            "**Gateway asenkron işleri:** {count}"
+    background_delegations: "**Arka plan delegasyonları:** {count}"
     none:                  "Aktif ajan veya çalışan görev yok."
     state_starting:        "başlatılıyor"
     state_running:         "çalışıyor"

locales/uk.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... і ще {count}"
     running_processes:     "**Фонові процеси, що виконуються:** {count}"
     async_jobs:            "**Асинхронні задачі гейтвея:** {count}"
+    background_delegations: "**Фонові делегування:** {count}"
     none:                  "Немає активних агентів або задач."
     state_starting:        "запускається"
     state_running:         "виконується"

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"

locales/zh.yaml

@@ -53,6 +53,7 @@ gateway:
     more:                  "... 还有 {count} 个"
     running_processes:     "**运行中的后台进程：** {count}"
     async_jobs:            "**网关异步任务：** {count}"
+    background_delegations: "**后台委派：** {count}"
     none:                  "没有活跃的代理或运行中的任务。"
     state_starting:        "启动中"
     state_running:         "运行中"