Add security reporting guidance

[KristianGrafana]

What changed

• Added guidance to the repository's agent instructions directing security issues to Grafana's security reporting page instead of the repository.

Why

This makes the expected reporting channel explicit for coding agents working in the repository.

Validation

• git diff --check
• Confirmed the commit changes only one agent instruction file with one added line.

[cla-assistant]

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}