Prepare 0.3.0-alpha.2 release

[Aaronontheweb]

Summary

• bump the prerelease suffix to alpha.2
• document the reviewed PowerShell proved-data receiver and alias-shadowing defenses from PR Prove PowerShell script block data safely #128
• record alpha.2 as pending until tag publication completes

Verification

• adversarial release review: PASS
• public API diff against published alpha.1: no changes for net8.0 or netstandard2.0
• dotnet build -c Release --no-restore
• dotnet test -c Release --no-restore (2575 passed)
• dotnet pack -c Release --no-restore -o ./bin/nuget
• package + symbols: ShellSyntaxTree.0.3.0-alpha.2
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• openspec validate v0-3-structured-shell-analysis --strict
• publish workflow verified to accept the bare 0.3.0-alpha.2 tag and matching release-note section