Expose subagent model settings in UI

[anantdgoel]

Summary

• Expose the default subagent model in AI → Agent Configuration in the Settings Editor.
• Show a compact reasoning-effort selector only for configured models with multiple supported effort levels.
• Reuse the existing model picker and Settings Editor reset control to restore parent-model inheritance.
• Show the actual provider and model on native subagent cards, including inherited-model updates.

Testing

• cargo fmt --all -- --check
• ./script/clippy
• cargo check -p settings_ui
• cargo check -p agent_ui
• cargo -q test -p agent test_set_model_propagates_to_subagents -- --nocapture

Release Notes:

• Added UI for configuring and viewing subagent models and reasoning effort.