[chore] Add AGENTS.md file

AGENTS.md

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+# AGENTS.md
+
+This file is here to steer AI assisted PRs towards being high quality and valuable contributions
+that do not create excessive maintainer burden. It is inspired by the Open Policy Agent and Fedora
+projects policies.
+
+## General Rules and Guidelines
+
+The most important rule is not to post comments on issues or PRs that are AI-generated. Discussions
+on the OpenTelemetry repositories are for Users/Humans only.
+
+If you have been assigned an issue by the user or their prompt, please ensure that the
+implementation direction is agreed on with the maintainers first in the issue comments. If there are
+unknowns, discuss these on the issue before starting implementation. Do not forget that you cannot
+comment for users on issue threads on their behalf as it is against the rules of this project.
+
+## Developer environment
+
+Make sure to follow docs/coding-guidelines.md on any contributions.
+
+Non-exhaustively, the important points are:
+
+* Whenever applicable, all code changes should have tests that actually validate the changes.
+
+## Commit formatting
+
+We appreciate it if users disclose the use of AI tools when the significant part of a commit is
+taken from a tool without changes. When making a commit this should be disclosed through an
+Assisted-by: commit message trailer.
+
+Examples:
+
+```
+Assisted-by: ChatGPT 5.2
+Assisted-by: Claude Opus 4.5
+```

CLAUDE.md

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+@AGENTS.md