ci: add GitLab Duo code review instructions
Summary
Add .gitlab/duo/mr-review-instructions.yaml with custom review instructions for GitLab Duo Code Review, tailored to LabKit's architecture and AGENTS.md guidelines.
What's in this MR?
Duo will automatically apply these instructions when reviewing MRs, focusing on higher-order concerns that linters cannot catch:
- LabKit v2 Architecture: interface design, component lifecycle, concurrency safety, error handling, security, logging
- LabKit v2 Tests: isolation patterns, testify conventions, table-driven tests
- LabKit v1 Root Module: backward compatibility only, no new API surface
- Shell Scripts: strict mode, quoting, structure
- GitLab CI Configuration: DAG patterns, rules syntax, two-module testing
- Markdown Documentation: LabKit README conventions, writing style
- Dependency Management: two-module go.mod structure, minimize deps
- Conventional Commits: scopes, header length
Instructions are derived from the project's AGENTS.md and adapted from common patterns used in other GitLab infrastructure projects.