Feature Request: Support Include Directive in mr-review-instructions.yaml

Problem Statement

To share and leverage experience with GitLab Duo custom configuration, teams need a way to maintain review instructions in a central repository and reference them from multiple projects. In particular, it should be possible to share different review instructions for different file-types, and for projects to decide which instructions they want to include/inherit/reuse, and which they want to define project-specifically.

Proposed Solution

Add support for an include directive in mr-review-instructions.yaml, similar to the existing functionality in .gitlab-ci.yml.

Use Case

Organizations could collect and optimize review instructions for specific file types customized to their standards and applications. These could be maintained in a centralized project (e.g., ai-adoption/gitlab-duo) with versioned instruction files such as review-instructions/python-review.yaml.

Other projects could leverage these instructions and automatically benefit from refinements:

In project group/product, file .gitlab/duo/mr-review-instructions.yaml

include:
  - project: "ai-adoption/gitlab-duo"
    file: "review-instructions/python-review.yaml"
    ref: 1.0.1

# Project-specific overrides can follow
...

Ideally it is possible to include snippets for certain file types, to have a pick-and-mix approach, e.g.

instructions:
  - include:
    - project: "ai-adoption/gitlab-duo"
      file: "review-instructions/python-review.yaml"
      ref: 1.0.1
    - project: "ai-adoption/gitlab-duo"
      file: "review-instructions/dockerfile-review.yaml"
      ref: main
  - name: Documentation Files
    fileFilters:
      - "**/*.md"
    instructions:
      ...
# Project-specific overrides can follow
...

where python-review.yaml might look like this:

  - name: Python Files
    fileFilters:
      - "**/*.py"
    instructions:
      ...