Add AGENTS.md for AI coding assistant guidance

[MenKNas]

• This PR follows the Contribution Guide
• Changelog updated
• Upgrade guide entry added

Description / Motivation

This PR adds AGENTS.md at the repo root to orient AI coding assistants working on the Content SDK monorepo. The file covers project structure, quick commands, package selection, DOs/DON'Ts, boundaries, example tasks, Git workflow, and links to existing guidance (CLAUDE.md, .cursor/rules/). It also updates the README AI Development Support section to reference AGENTS.md.

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate)

Types of changes

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

[github-actions]

📦 Package Size and Test Coverage Report

Package	Base Size	PR Size	Δ Change	Base Coverage	PR Coverage	Δ Change
analytics-core	122.52 KB	122.52 KB	✅ 0.00 KB	100.00%	100.00%	0.00%
cli	58.70 KB	58.70 KB	✅ 0.00 KB	69.73%	69.73%	0.00%
content	416.55 KB	416.55 KB	✅ 0.00 KB	92.05%	92.05%	0.00%
core	102.67 KB	102.67 KB	✅ 0.00 KB	92.76%	92.76%	0.00%
create-content-sdk-app	244.96 KB	280.71 KB	🔺 +35.75 KB	96.22%	96.22%	0.00%
events	76.91 KB	76.91 KB	✅ 0.00 KB	98.40%	98.40%	0.00%
nextjs	430.77 KB	430.77 KB	✅ 0.00 KB	89.76%	89.76%	0.00%
personalize	63.45 KB	63.45 KB	✅ 0.00 KB	99.32%	99.32%	0.00%
react	288.02 KB	288.02 KB	✅ 0.00 KB	93.85%	93.85%	0.00%
search	7.30 KB	7.30 KB	✅ 0.00 KB	98.57%	98.57%	0.00%
Total	—	—	🔺 +35.75 KB	—	—	—