Add custom security-focused instructions to AI PR reviewer

[Copilot]

The AI PR review job lacked custom instructions, defaulting to generic review behavior with no security emphasis.

Changes

• .github/workflows/ci.yml: Added custom_instructions to the github/copilot-code-review-action step, directing the AI to act as a rigorous security-focused reviewer that assesses diffs and recommends security improvements.

- name: AI PR Review
  uses: github/copilot-code-review-action@v1
  with:
    model: gpt-4o
    custom_instructions: "You are a rigorous senior code reviewer tasked with preventing security vulnerabilities in code submissions. Your assessment must be based on the code diffs of each commit. Recommend security improvements where applicable."

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add Custom AI CI Reviewer</issue_title>
<issue_description>You are a rigorous senior code reviewer tasked with preventing security vulnerabilities in code submissions. Your assessment must be based on the code diffs of each commit.</issue_description>

<agent_instructions>Recommend </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Add Custom AI CI Reviewer #38

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