Fix primary agent model precedence

[afafara]

Summary

• let Atlas and Sisyphus keep the runtime-selected model when agent config or category overrides also define a model
• restore the resolved model and variant after applying overrides so primary-agent model selection is not clobbered
• add regression coverage for direct model overrides and category-based overrides in src/agents/utils.test.ts

Testing

• bun test src/agents/utils.test.ts

---

Summary by cubic

Fix model precedence for primary agents so the UI-selected model always wins over agent and category overrides. Atlas and Sisyphus now keep the runtime-selected model and restore the resolved variant when applicable.

• Bug Fixes
  • Preserve uiSelectedModel for atlas and sisyphus even when overrides specify a model or category.
  • After applying overrides, re-apply the resolved model and variant (if any) to prevent clobbering.
  • Add regression tests for direct model overrides and category-based overrides to lock precedence.

^{Written for commit d62cdfe. Summary will update on new commits.}

[github-actions]

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[afafara]

I have read the CLA Document and I hereby sign the CLA

[afafara]

recheck

[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Requires human review: This PR deliberately flips model precedence logic, which is a functional change that alters existing behavior and breaks previous test expectations.}

[kithawk]

I have read the CLA Document and I hereby sign the CLA