make variant modal less annoying

[Hona]

Summary

• remember the variant choice per model, including choosing the default
• only prompt for a variant once per model; after that ctrl+t cycles variants and /variants opens the modal manually

[Copilot]

Pull request overview

Updates the TUI’s model-variant UX so variant selection is remembered per model and the variant picker modal is shown only when needed, with keyboard cycling support thereafter.

Changes:

• Persist a per-model variant “selection” (including an explicit “default” choice) and add validation for the active/usable variant.
• Add an explicit “Default” option to the variant selection dialog and adjust model selection flow to avoid re-prompting once a choice is recorded.
• Introduce a dedicated “Variant cycle” command/action and add /variants to manually open the variant modal.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/opencode/src/cli/cmd/tui/context/local.tsx	Adds selected() vs validated current() variant accessors; persists "default" sentinel for explicit default selection; updates cycling logic accordingly.
packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx	Switches prompt submission to use the stored variant selection value.
packages/opencode/src/cli/cmd/tui/component/dialog-variant.tsx	Adds an explicit “Default” entry and uses selected() to highlight the stored choice.
packages/opencode/src/cli/cmd/tui/component/dialog-model.tsx	Avoids showing the variant modal if a prior selection exists for that model; otherwise prompts once when variants are available.
packages/opencode/src/cli/cmd/tui/app.tsx	Adds a new “Variant cycle” action and introduces /variants for manual modal access.

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