Implement automated CI failure investigator

[shralex]

This PR implements a Gemini-powered automated investigator to diagnose CI test failures in the repository. It enables the @gemini-cli /investigate command and runs automatically on pipeline failures to provide actionable diagnostic feedback.

Key Features:

• Automated Failure Diagnostics: Formulates a systematic diagnostics report containing the specific test flavor/jobs that failed, exact traceback logs, and logical context of the failure.
• Diagnostics Protocol: Guides the model to perform efficient diagnostics—starting with log parsing, searching git history for flakes, and analyzing recent code changes in the PR or main branch.
• Targeted Fix Recommendations: Directs the model to generate high-confidence code diff suggestions when the root cause is identified.
• Integrated Reporting: Automatically writes the final markdown report to .gemini/findings.md and posts it as a comment directly to the target PR or failure issue.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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