Prepare 0.3.0-alpha.5 release

[Aaronontheweb]

Summary

• package the merged host-selected PowerShell dialect and language-boundary slice
• add explicit PowerShell 7.6 and Windows PowerShell 5.1 release guidance
• preserve the v0.2 compatibility projection and document the two additive API changes
• prepare the prerelease used by Netclaw's native-Windows integration gate

Validation

• dotnet tool restore
• dotnet restore
• dotnet build -c Release --no-restore — clean, zero warnings
• dotnet test -c Release --no-build — 2,815 passed
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• dotnet pack -c Release --no-build -o ./bin/nuget-alpha5
  • ShellSyntaxTree.0.3.0-alpha.5.nupkg
  • ShellSyntaxTree.0.3.0-alpha.5.snupkg
  • net8.0 and netstandard2.0
• publish-workflow release-note extraction reproduced locally
• slopwatch analyze -d . --fail-on warning --no-baseline — 0 findings
• openspec validate v0-3-structured-shell-analysis --strict
• git diff --check
• adversarial release review: PASS

Publication remains intentionally unchecked in IMPLEMENTATION_PLAN.md until the reviewed merge is tagged and the tag workflow proves NuGet plus GitHub release delivery.