Add experiment results to GitHub job summaries

[Copilot]

Experiment runs uploaded results.json as an artifact but did not surface the formatted results in the Actions job summary. This makes run outcomes visible directly from the workflow page.

• Job summary output
  • Appends the existing formatted experiment results table to $GITHUB_STEP_SUMMARY when available.
  • Preserves existing console output and results.json artifact generation.

await fs.appendFile(GITHUB_STEP_SUMMARY, `## Experiment results\n\n\`\`\`\n${resultSummaries}\n\`\`\`\n`)

[Copilot]

Pull request overview

This PR surfaces experiment result summaries directly in GitHub Actions job summaries while preserving the existing console output and results artifact flow.

Changes:

• Reads GITHUB_STEP_SUMMARY from the environment.
• Adds a helper to append formatted experiment results to the Actions job summary when available.
• Reuses the formatted result summary for both console output and summary output.

Show a summary per file

File	Description
packages/agent-eval/src/cli.ts	Adds GitHub job summary output for formatted experiment results.

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