fix(app): reflect configured reasoning effort in web ui

[kdybicz]

Issue for this PR

Closes #17588

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The web UI did not reflect the reasoning/thinking effort configured through model options in opencode.json.

For configs like:

{
  "provider": {
    "openai": {
      "models": {
        "gpt-5.4": {
          "options": {
            "reasoningEffort": "high"
          }
        }
      }
    }
  }
}

New sessions still showed the variant selector as Default, and the first user message was sent without a variant. That also meant refresh could not preserve the selected thinking effort from session history.

This change makes the app infer the UI variant from configured model options when no explicit session or agent variant is set. It matches the effective reasoning/thinking fields used by provider variants, so provider-added defaults like reasoningSummary do not block the mapping.

This keeps the existing precedence intact:

• explicit session selection wins
• explicit Default still overrides configured inference
• agent-configured variant still wins over model-option inference

How did you verify your code works?

• Ran bun test src/context/model-variant.test.ts in packages/app
• Ran bun typecheck in packages/app
• Verified locally with:
  • backend: bun run --conditions=browser ./src/index.ts serve --port 4096 from packages/opencode
  • app: bun dev -- --port 4444 from packages/app
• Confirmed a new session shows the configured High effort in the UI when reasoningEffort: "high" is set in config

Screenshots / recordings

Not added

Checklist

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

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