Release ShellSyntaxTree 0.4.0-beta.1

[Aaronontheweb]

Summary

• set the package version to 0.4.0-beta.1
• promote the reviewed Unreleased notes with the release date
• update the README package command and 0.4 version summary
• record that implementation PR Add PowerShell approval-fatigue facts #173 merged while publication and Netclaw adoption remain pending

Validation

• dotnet build -c Release: clean
• dotnet test -c Release: 3,336 passed
• dotnet pack -c Release: package and symbols package produced
• package metadata reports 0.4.0-beta.1 for netstandard2.0 and net8.0
• copyright headers: passed
• NuGet vulnerability audit: clean
• git diff --check: passed

After this PR merges, the bare tag 0.4.0-beta.1 will trigger the existing trusted-publishing workflow. No Netclaw version or release is part of this PR.