docs(readme): Add CI runner health check marker

[henryvo117]

What does this PR do?

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[github-actions]

AI review by Codex (OpenAI) - workflow run

patch is correct — the change only adds a harmless HTML comment to the README and does not affect runtime behavior.

[datadog-datadog-us1-prod]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 44.74% (-7.98%)

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