feat(kanban): add auto_promote_children config toggle

When the kanban auto-decomposer fans a triage task into child tasks,
recompute_ready() immediately promotes parent-free children to 'ready'
so the dispatcher picks them up. Some users want a manual workflow
where children stay in 'todo' for review before dispatch.

Add 'kanban.auto_promote_children' config key (default: true):
- false: children stay in 'todo' after decomposition
- true: existing behavior (auto-promote to 'ready')

Changes:
- kanban_db.py: decompose_triage_task() gains auto_promote param
- kanban_decompose.py: reads auto_promote_children from config
- kanban dashboard API: exposes the new setting in GET/PUT /orchestration

Closes #28016

plugins/kanban/dashboard/plugin_api.py

@@ -1701,6 +1701,7 @@ class OrchestrationSettingsBody(BaseModel):
     orchestrator_profile: Optional[str] = None
     default_assignee: Optional[str] = None
     auto_decompose: Optional[bool] = None
+    auto_promote_children: Optional[bool] = None
 
 
 @router.get("/orchestration")
@@ -1716,6 +1717,7 @@ def get_orchestration_settings():
     explicit_orch = (kanban_cfg.get("orchestrator_profile") or "").strip()
     explicit_default = (kanban_cfg.get("default_assignee") or "").strip()
     auto_decompose = bool(kanban_cfg.get("auto_decompose", True))
+    auto_promote_children = bool(kanban_cfg.get("auto_promote_children", True))
 
     # Resolve fallbacks the same way the decomposer does.
     resolved_orch = explicit_orch
@@ -1738,6 +1740,7 @@ def get_orchestration_settings():
         "orchestrator_profile": explicit_orch,
         "default_assignee": explicit_default,
         "auto_decompose": auto_decompose,
+        "auto_promote_children": auto_promote_children,
         "resolved_orchestrator_profile": resolved_orch,
         "resolved_default_assignee": resolved_default,
         "active_profile": active_default,
@@ -1803,6 +1806,9 @@ def set_orchestration_settings(payload: OrchestrationSettingsBody):
     if payload.auto_decompose is not None:
         kanban_section["auto_decompose"] = bool(payload.auto_decompose)
 
+    if payload.auto_promote_children is not None:
+        kanban_section["auto_promote_children"] = bool(payload.auto_promote_children)
+
     try:
         save_config(cfg)
     except Exception as exc: