Fix TaskHub OpenCode model override handling

[muqiao215]

Summary

• preserve explicit OpenCode task models during TaskHub runtime pre-resolution
• add a regression test for tasks that target OpenCode while the foreground runtime uses another provider

Problem

On hosts like racknerd-436b0c0, foreground opencode is usable, but background tasks fail with error:opencode_default_model_unresolved even when the task entry already carries model: zhipuai/glm-5.1.

Root cause: TaskHub._run() special-cased opencode by calling resolve_runtime_provider_target("opencode", ""), which discarded the explicit task model and forced default-model discovery.

Testing

• pytest -q tests/tasks/test_hub.py tests/cli/test_service_extended.py -q