fix(ui): show model with subagent thinking level

[xz-dev]

Summary

• always retain the effective model name for Agent UI rendering, including runs that inherit the parent model
• show the model together with its model-dependent thinking level in collapsed and expanded Agent results and in ConversationViewer
• keep model names safe for single-line terminal rendering

Why

A thinking level such as max has little meaning without the model it applies to. The UI previously hid the model name whenever a subagent used the same model as its parent session. Because the UI gave no indication that the model was inherited, users only saw that model names appeared for some runs and disappeared for others, leaving a bare thinking level with no visible model context.

Tests

• npm run lint
• npm run typecheck
• npm run test — 749 passed, 5 skipped
• npm run build