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

[itsmeadi]

Summary

• Add AGENTS.md with gem layout, Makefile/CI jobs, OpenAPI regen, and RubyGems release rules (feat:/fix:/!:).

Test plan

• Skim AGENTS.md against .github/workflows/ and README.md Release Process
• Confirm docs: title does not trigger release.yml publish

Made with Cursor

[mogita]

No merge conflict, but the ## Release section is now stale: #86 replaced scripts/release/bump_version.rb with release-please. bug: no longer releases, the dispatch inputs are one publish_tag instead of version_bump/use_current_version, the bump lands in a bot Release PR rather than a local tagging commit, and merging that Release PR is what cuts the release. ## CI needs a touch too: ci.yml is pull_request only now and the jobs moved into a reusable run_tests.yml.

Source of truth is the Release Process section of README.md. getstream-go#161 has the same section written against this flow if you want a template.