fix(cli): allow clearing agent variant overrides

[marius-kilocode]

The Agent Behaviour model and variant override dropdowns added in #10505 introduced a settings-save regression. Selecting Not set for an agent model correctly sends model: null and variant: null so both overrides are removed, but the CLI config schema accepted the model deletion sentinel while rejecting the variant deletion sentinel. VS Code therefore could not save the reset to the default model.

Allow nullable agent variants in config patches so model and variant overrides can be cleared consistently. Keep the deletion behavior scoped to config persistence, including JSONC updates that preserve unrelated agent settings and existing comments.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (3 files)

• .changeset/calm-agent-variant-clear.md
• packages/opencode/src/config/agent.ts
• packages/opencode/test/kilocode/config/config.test.ts

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_{Reviewed by claude-4.6-sonnet-20260217 · 307,567 tokens}

_{Review guidance: REVIEW.md from base branch main}