feat(kanban): surface per-task model_override in show + tool output

Salvages #26897 by @loicnico96. The per-task model_override DB column
already exists on main, but it wasn't exposed in user-facing surfaces.
This adds:
- 'kanban show' prints 'model: <name>' when model_override is set
- kanban_show / kanban_list tool responses include the model_override field

Original branch was stale (PR was authored against an older field name
'model'); applied the substantive surface exposure manually using the
current 'model_override' field name.

tools/kanban_tools.py

@@ -232,6 +232,7 @@ def _task_summary_dict(kb, conn, task) -> dict[str, Any]:
         "started_at": task.started_at,
         "completed_at": task.completed_at,
         "current_run_id": task.current_run_id,
+        "model_override": task.model_override,
         "parents": parents,
         "children": children,
         "parent_count": len(parents),
@@ -275,6 +276,7 @@ def _handle_show(args: dict, **kw) -> str:
                     "completed_at": t.completed_at,
                     "result": t.result,
                     "current_run_id": t.current_run_id,
+                    "model_override": t.model_override,
                 }
 
             def _run_dict(r):