Add repository AGENTS guide

[opensourcevk]

Summary

• add a root AGENTS.md with repository-specific guidance for working in opentelemetry-java
• document module layout, build and test entry points, style expectations, and repo-specific constraints

Testing

• not run (docs-only change)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.30%. Comparing base (839f235) to head (f7466f9).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8174   +/-   ##
=========================================
  Coverage     90.29%   90.30%           
- Complexity     7650     7651    +1     
=========================================
  Files           843      843           
  Lines         23059    23061    +2     
  Branches       2309     2310    +1     
=========================================
+ Hits          20822    20826    +4     
+ Misses         1519     1516    -3     
- Partials        718      719    +1     

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[jack-berg]

This reads like https://github.com/open-telemetry/opentelemetry-java/blob/main/CONTRIBUTING.md.

Can we not do a pass to modernize CONTRIBUTING.md and have a minimum AGENTS.md that redirects to CONTRIBUTING.md?

Things I don't want:

• Maintain two copies of the same information
• Prioritize machines over humans

[opensourcevk]

This reads like https://github.com/open-telemetry/opentelemetry-java/blob/main/CONTRIBUTING.md.

Can we not do a pass to modernize CONTRIBUTING.md and have a minimum AGENTS.md that redirects to CONTRIBUTING.md?

Things I don't want:

• Maintain two copies of the same information
• Prioritize machines over humans

updated agents.md

[breedx-splk]

reasonable starting point to me.

[opensourcevk]

@jack-berg may i kindly request approval and merge please?

[otelbot]

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