Draft: docs: add AGENTS.md documentation

Relates to issue #584017

Created with Duo: Developer Flow in Session ID: 1404201

Changes

This MR implements comprehensive documentation for AGENTS.md custom instructions and creates a unified overview of all GitLab Duo customization options.

1. Updated Configuration Paths

• Updated doc/user/gitlab_duo_chat/agentic_chat.md to use new user configuration directory paths:
  • macOS/Linux: ~/.gitlab/duo/chat-rules.md (previously glab-cli)
  • Windows: %APPDATA%\GitLab\duo\chat-rules.md (previously %LOCALAPPDATA%\glab-cli)

2. Added AGENTS.md Documentation

• Created comprehensive AGENTS.md section in doc/user/gitlab_duo_chat/agentic_chat.md including:
  • Introduction explaining IDE-only support
  • Three levels of AGENTS.md files (user-level, workspace-level, subdirectory-level)
  • Clear explanation of how Chat combines instructions from multiple files
  • Supported environments subsection (VS Code, JetBrains, Visual Studio, Duo CLI)
  • Version numbers marked as "TBD - to be confirmed with engineering team"
  • Practical examples for 4 diverse use cases:
    1. Beginner-friendly explanations
    2. Team coding standards
    3. Monorepo-specific context
    4. Security-focused review guidelines
  • Comparison table between AGENTS.md and chat-rules.md with precedence rules

3. Created Unified Customization Overview

• Created new file: doc/user/gitlab_duo/customize_duo.md with:
  • Introduction to all customization options
  • Decision matrix table comparing AGENTS.md vs chat-rules.md vs mr-review-instructions.yaml
  • Precedence rules section (5-level hierarchy)
  • Quick start guides for different use cases
  • Best practices section
  • Related topics links
  • Proper metadata and history (introduced in GitLab 18.8)

4. Added Navigation and Cross-Links

• Added "Customizing GitLab Duo" section to doc/user/gitlab_duo/_index.md
• Added cross-reference links in doc/user/gitlab_duo_chat/agentic_chat.md:
  • After "Create custom rules" section
  • After "Create AGENTS.md instruction files" section
• Added cross-reference in doc/user/project/merge_requests/duo_in_merge_requests.md after best practices

5. Documentation Structure

All documentation follows GitLab's established patterns:

• Proper metadata (stage, group, info, title)
• History sections with GitLab issue/MR links
• Consistent formatting and structure
• Cross-linking for discoverability

Notes

Tasks 12-13 regarding Duo CLI documentation were skipped as the GitLab CLI documentation has been moved to an external site (https://docs.gitlab.com/cli/) and is not available in this repository.