docs: add AGENTS.md for structure, CI, and release

[itsmeadi]

Summary

• Add AGENTS.md with PHP layout, Make/CI, OpenAPI regen, and Packagist release (conventional title + ticket).

Test plan

• Skim AGENTS.md against .github/workflows/ and README Release Workflow
• Confirm docs: title does not trigger a Packagist publish

Made with Cursor

[mogita]

No merge conflict, but the ## Release section is now stale: #65 replaced scripts/release/bump_version.php with release-please. bug: no longer releases, the dispatch inputs are one boolean resync_packagist instead of version_bump/use_current_version/prerelease, the bump lands in a bot Release PR rather than a commit pushed to the default branch, and merging that Release PR is what tags and announces to Packagist. ## CI needs a touch too: ci.yml is pull_request only now and the test job moved into a reusable run_tests.yml.

Source of truth is the Release Process section of README.md, and .github/RELEASE_SETUP.md was repointed in the same PR. getstream-go#161 has the same section written against this flow if you want a template.