chore: add AGENTS.md to guide AI-assisted contributions

[MikeGoldsmith]

Description

Adds AGENTS.md to guide AI agents making contributions to this repository. Modelled after the equivalent file in opentelemetry-python-contrib#4233.

The file covers:

• A link to the OTel community GenAI policy
• The rule against posting AI-generated comments on issues/PRs
• PR scoping guidance (no opportunistic cleanup)
• Repo structure overview
• Common commands for linting and testing
• Coding guidelines
• Commit Assisted-by: trailer convention

Also adds CLAUDE.md as a pointer to AGENTS.md for Claude-based tooling.

Type of change

• 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 not work as expected)
• This change requires a documentation update

How Has This Been Tested?

N/A — documentation only.

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR: Add AGENTS.md with project structure and commands opentelemetry-python-contrib#4233

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated