fix(tui): default to configured agent variants - #47076

Summary

Use the selected agent's configured model variant as the default for a fresh TUI session, ahead of the saved per-model preference. Previously the new-session model copy discarded the agent's variant.

• Keep manual variant choices editable, including an explicit default/no-variant choice.
• Preserve overrides independently for each agent and location, and retain the current variant when reselecting the same model.
• Continue using saved preferences when no agent variant is configured or when selecting another model.
• Leave existing sessions' durable selections unchanged.

This change is scoped to the full TUI. Mini and ACP startup behavior is unchanged.

Validation

From packages/tui:

bun test test/context/local-variant.test.tsx test/context/local.test.ts test/model-preference.test.ts test/mini/variant.shared.test.ts test/cli/tui/dialog-session-list.test.tsx
bun typecheck

• 14 focused tests pass, with 75 assertions; typecheck, formatting, and diff checks pass.
• The real LocalProvider regression test failed before the fix for both a conflicting saved preference and no saved preference.
• Tested the real TUI on Linux in a 112×34 terminal with the dark OpenCode theme, isolated HOME/XDG directories, a configured local fixture model, and no inference requests.
• Before f98a6286da8e496e9287e4dbe36b6f9a7153c7a4: agent configured low, saved preference medium, footer shows medium.
• After 104b483bb9a347c5407d845bd8bffa16134b1bde: footer starts at low; choosing high through the variant picker works; cycling to default remains unset after changing Plan's variant and returning to Build. Prompt focus remains editable.

Requested by: @rekram1-node (Aiden via Slack)