docs: add AGENTS.md to help AI coding tools understand the repo

[OwnOptic]

Summary

Closes #710

Adds an AGENTS.md file to the root of the repository. This is a standard convention that helps AI coding assistants (GitHub Copilot, Claude Code, Cursor, etc.) understand the repository structure, conventions, and workflows without needing to explore the entire codebase.

What's included

• Repository structure overview (docs/, src/, mkdocs.yml, etc.)
• Lab naming convention table (E, BMA, BAF, MCS, MAB, MSA prefixes explained)
• How to run the docs site locally (mkdocs serve + Docker alternative)
• How to add or edit lab content (nav registration, snippets, admonitions)
• JavaScript files and their purpose (cc-award.js, cc-next.js, etc.)
• i18n guidance for Japanese translations
• Contribution checklist (CLA, no secrets, mkdocs serves clean)
• Key links (issues, discussions, awards, recognition program, sample gallery)

Test plan

• mkdocs serve still runs without errors (no change to docs content)
• File renders cleanly on GitHub (tables, code blocks, headings)

[OwnOptic]

@microsoft-github-policy-service agree [company="{Witivio}"]

[OwnOptic]

@microsoft-github-policy-service agree

[rabwill]

Thank you for the suggestion, the team as a whole will look into significant changes in the repo. Holding this off for now.