fix(opencode): scope TUI variant state per agent

[CptnKirk]

Issue for this PR

Closes #13718
Refs #16034

Type of change

• Bug fix

What does this PR do?

Fixes a TUI bug where persisted variant selections were keyed by provider/model, so agents that shared a model could inherit a stale variant selection from each other.

This changes TUI variant persistence to use agent-scoped keys instead, and resolves the active variant as selected > configured > none. Obsolete legacy provider/model entries are ignored on load and removed on the next save.

How this relates to #16034

This proposed PR fixes the #13718 persistence-keying bug by storing TUI variant state per agent instead of per provider/model.

#16034 describes the broader symptom. #16032 approaches it by reapplying configured variants during agent/session switching; this proposed PR addresses the underlying cross-agent persistence collision instead.

How did you verify your code works?

Added unit tests in packages/opencode/test/cli/tui/model-variant.test.ts covering:

• legacy key filtering
• configured variant precedence
• cycling behavior from configured and explicit default states

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[null-axiom]

I am experiencing the same issue this PR will fix. I am switching between plan and build agents, but the problem is that I cannot persist a variant exclusive to each agent, it currently shares the variant when switching between agents.

[tarurar]

Guys, please review and merge the PR, it fixes real bug.