[QUESTION] Agent default variant handling in TUI/Desktop (#7156)

[CasualDeveloper]

Question

Would maintainers accept the remaining narrow client-side scope for agent-configured model variants in #7156?

The PR now only covers:

• app/TUI local-state and hydration behavior for selected vs configured variants
• shared helper/tests for selected / Default / agent-configured resolution
• prompt bar display of the effective variant

It intentionally does not add backend/session/API/SDK behavior. The broader backend pieces from the original #7138 discussion have already landed or are out of scope for this PR.

This is a replacement for auto-closed #7138 so #7156 stays linked to an open design/review thread.

[CasualDeveloper]

Rebased #7156 onto the latest dev again and updated the branch to match current semantics.

Verified locally with bun turbo typecheck, bun --cwd packages/opencode test --timeout 900000 ./test/agent/agent.test.ts, and bun --cwd packages/shared test --timeout 900000 ./test/util/model-variant.test.ts.

If maintainers want a different direction, happy to rework it.