fix: default SDK eval model fallback to grok-code-fast

[zhao0112]

Summary

• Fixes a regression where the eval runner default model could be opencode/grok-code, which is currently not available.
• Changes evals/framework/src/sdk/test-runner.ts fallback default to opencode/grok-code-fast to match the CLI’s documented default and current provider availability.

Related Issue

• [BUG] Default eval model does not exist but the eval suite still runs in full. #222

Validation

• Requested independent review via requesting-code-review workflow style (delegate reviewer): passed.
• Verified diff is a single-line default-model fallback update.
• Ran focused tests:
  • npx vitest run src/__tests__/error-handling.test.ts src/__tests__/framework-confidence.test.ts
    • Result: 5 existing failures in repository baseline tests; none were from touched path (pre-existing).
  • npm run build in evals/framework currently reports pre-existing TS issues in integration/evaluator areas.
  • npm run eval:sdk -- --help currently fails on an existing test data issue (invalid enum value in a repository test fixture).

Notes

• No schema/API changes.
• No new dependencies added.