chore: consolidate AI assistant instructions into AGENTS.md

[taegyunkim]

Description

I believe these should be read by both claude/codex automatically on startup. As far as I know they don't support the following as cursor does.

globs:
alwaysApply: true

When I ask the following on main

❯ Don't read any file but try to answer the following using whatever you know about the project


  What does Critical Architecture Decisions section say from our dd-trace-py.mdc.

⏺ I don't have the contents of .cursor/rules/dd-trace-py.mdc in my current context or memory. The AGENTS.md file references it as one of the rule files to read, but doesn't inline its contents.

  I'd need to read the file to give you an accurate answer. Want me to go ahead and read it?

Given that such globs: only work for cursor, I updated Domain Reference Guides section to list proper .cursor/rules/*.mdc files with directories that are relevant. Though this still doesn't guarantee that these tools read those files, but they would try their best.

Testing

Risks

Additional Notes

[cit-pr-commenter-54b7da]

Codeowners resolved as

AGENTS.md                                                               @DataDog/apm-core-python