--model was accepted by parse_args but only consumed inside
handle_once_mode, so `python aibox.py --model kimi` (server mode)
silently ignored the override.
The model now flows as a process-wide override:
main() -> EnhancedAIManager(model=...) -> DeepSeekChat(model=...) in
all three AI creation paths: add_ai (main dialogue),
WebSocketHandler._create_session_ai (per-connection sessions), and
TaskScheduler._create_task_ai (scheduled tasks). Manager logs
"CLI 模型覆盖已启用" at startup for observability. --once mode
plumbing is unchanged; --model=None keeps config-driven behavior.
Tests: new TestH3ModelPlumbing (6 cases) mocking DeepSeekChat to
assert the constructor kwarg across manager/session/task paths plus
parse_args; full suite 288 passed.