Add AGENTS.md

[rombert]

Description

Adds a basic AGENTS.md file and a Claude Code compat CLAUDE.md.

Related Issue

#1305

Motivation and Context

Better support for coding agents.

How Has This Been Tested?

Local coding agent explorations.

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[rombert]

Draft since I need to add information for the on-prem/Adobe Managed Services variant and take into account variations (dispatcher, different frontend technologies, add-ons).

[rombert]

@bpauli / @vladbailescu - can you help me figure out who can review this PR?

[rombert]

(please squash the commits when merging)