Add AGENTS.md with project structure and commands (#4233)

* Add CLAUDE.md with project structure and commands

* Add AGENTS.md symlink to CLAUDE.md

* Move guidance to AGENTS.md and address review feedback

- Move main content from CLAUDE.md to AGENTS.md so all AI agents
  (not only Claude) pick up the guidance; CLAUDE.md now just references
  it via `@AGENTS.md`.
- Add general rules, PR scoping, and `Assisted-by:` commit trailer
  guidance (inspired by the Collector's AGENTS.md).
- Clarify that only instrumentation packages live under
  `src/opentelemetry/instrumentation/{name}/`; other package types use
  their own namespace.

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>