feat: support extra maintainers files with specific check for readme.md [CM-1032]

[mbani01]

This pull request enhances the maintainer file detection logic in the MaintainerService by broadening the list of recognized maintainer-related files and improving how README.md files are handled. The main goal is to make maintainer identification more accurate and robust.

Improvements to maintainer file detection:

• Expanded the list of known maintainer files to include GOVERNANCE.md, README.md, and SECURITY-INSIGHTS.md in addition to existing files.
• Updated the logic to skip README.md files that do not contain maintainer-related content, reducing false positives when searching for maintainer information.

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Note

Medium Risk
Changes maintainer-file selection logic, which can alter which repositories yield maintainer data and may inadvertently skip valid README-based ownership info if it doesn’t contain the keyword check.

Overview
Improves maintainer file detection by expanding the known candidate list to include GOVERNANCE.md, README.md, and SECURITY-INSIGHTS.md.

Adds a guard to skip README.md when it doesn’t contain “maintainer” text, applying this both to direct filename matching and to AI-suggested files to reduce false positives.

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