ci: add tag release asset workflow

[mcoder2014]

Summary

• Add a GitHub Actions release workflow triggered by v* tags.
• Build Go CLI zip assets for linux/darwin amd64/arm64, package video_subtitle as a source zip, and package the Emby plugin DLL as a dedicated zip.
• Add docs/release.md, README entry, and AGENTS release maintenance rules.

Behavior

• Pull requests run the same tests and release asset packaging as a dry-run.
• Only tag pushes run the final gh release create / gh release upload step.
• Release assets include checksums.txt.

Verification

• git diff --check
• go test ./...
• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest video_subtitle/video_subtitle_test.py
• PYTHONDONTWRITEBYTECODE=1 python3 -m py_compile video_subtitle/video_subtitle.py video_subtitle/video_subtitle_test.py video_subtitle/lib/*.py
• dotnet test emby_plugins/video_subtitle/LifeTools.Emby.VideoSubtitle.sln --configuration Release
• dotnet build emby_plugins/video_subtitle/LifeTools.Emby.VideoSubtitle.sln --configuration Release
• Static workflow checks for tag trigger, PR dry-run, contents write permission, and tag-only release step
• Local smoke packaging for linux/amd64 Go zip, video_subtitle source zip, Emby plugin zip, and checksums

Note

actionlint is not installed on this machine, so full GitHub Actions linting is left to the PR dry-run on GitHub.