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.

hermes_cli/kanban.py

@@ -1319,6 +1319,8 @@ def _cmd_show(args: argparse.Namespace) -> int:
           (f" @ {task.workspace_path}" if task.workspace_path else ""))
     if task.skills:
         print(f"  skills:    {', '.join(task.skills)}")
+    if task.model_override:
+        print(f"  model:     {task.model_override}")
     # Effective retry threshold. Show the per-task override if set,
     # otherwise the dispatcher's resolved value from config (or the
     # default if config doesn't set it either). Helps operators see