Merge PR #42: build(release) publish every client on tag push from one version source

Adds .github/scripts/check-versions.py — single source of truth for all 7
version numbers across 5 files (herdi-win csproj, build.ps1, herdi-mac build.sh,
both plugin.toml). Fixes the drift that left plugin.toml at 0.5.0 for 4 releases.

Adds .github/workflows/release.yml to publish all clients on tag push.

Supersedes the ad-hoc sed version-sync. Thanks @nickrunning!