chore: add duo review instructions

What does this MR do and why?

Adds GitLab Duo MR review instructions for Rust files. Three review agents are configured:

• Rust Security: flags unsafe code, unvalidated input, hardcoded secrets, panic vectors (unwrap/expect on untrusted data), integer overflow, SQL injection, JWT validation, overly permissive access control, and DOS vectors
• Rust Performance: flags unnecessary allocations, inefficient iteration, missing zero-copy patterns, and excessive cloning
• Logging Security: flags sensitive data in logs (tokens, passwords, keys, PII, user content) and checks for log injection vulnerabilities

Related Issues

N/A

Testing

Configuration file follows GitLab Duo instructions format.

Performance Analysis

• This merge request does not introduce any performance regression. If a performance regression is expected, explain why.