maint: Add agent instructions for handbook contributions

Why is this change being made?

Agents need repository-specific context before editing the handbook. The existing Duo chat rules only covered Markdown spacing and two CI conventions, while build commands, content ownership, generated files, validation scope, and merge request preparation required manual discovery.

This MR adds a root AGENTS.md with concise guidance for navigating, editing, validating, and building the handbook. It folds in the existing .gitlab/duo/chat-rules.md instructions, points agents to the current Duo review rules and MR templates, and documents how to apply template labels through GitLab tooling.

The chat rules file is deleted after merging its guidance into AGENTS.md. Because this deletes a file under .gitlab/, please treat it as a potentially breaking maintainer-file change and review with @gitlab-com/content-sites/handbook-tools.

References docsy-gitlab#36

Validation

• npx markdownlint-cli2 AGENTS.md
• git diff --check main...HEAD

Author and Reviewer Checklist

• Provided a concise title for this merge request
• Explained why the change is proposed
• Assigned a handbook maintainer for review
• Shared the MR in #handbook if needed

Commits

One focused commit adds AGENTS.md and removes the merged chat rules.