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

[itsmeadi]

Summary

• Add AGENTS.md with .NET layout, make/CI, OpenAPI regen, and NuGet Trusted Publishing.

Test plan

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

Made with Cursor

[mogita]

#76 is merged, so ## Release and part of ## CI are now stale. What to change:

• Drop scripts/bump_version.sh; the bump lives in a bot Release PR, and merging that PR tags, packs and publishes.
• Drop bug: from the bump rules.
• Dispatch inputs are one publish_tag, not version_bump / use_current_version.
• ci.yml is pull_request only (main, master, *.x); its steps moved to a reusable run_tests.yml that the release gate also calls, so the suite runs before the tag.
• Tests and packing are .NET 8.0.x, not 9.0.x, and run in Release only.
• STREAM_API_KEY is vars then secrets in one place, so the CI-vs-release split is gone.

Unchanged: .nupkg artifacts on the GitHub Release. Source of truth is the Release Process section of README.md.