feat: add downstream README alignment workflow

[david-r-cushman]

Summary

• add the downstream README skeleton, cleanup workflow, and README alignment workflow to powershell-dev-template
• bring repo-local skills, ADRs, generated Markdown support, and guidance sync assets into parity with the pwsh template while preserving Windows PowerShell design boundaries
• update versioning, documentation, and test coverage for the new downstream README workflow surfaces

Issue Resolution

• Closes [Feature]: Turn the shared README skeleton into a downstream starter README #10
• Closes [Feature]: Review missing template-maintenance capabilities for parity with pwsh-dev-template #11

Validation

• git diff --check
• powershell.exe -NoProfile -File ./scripts/Test-VersionPolicy.ps1
• powershell.exe -NoProfile -File ./scripts/Test-TemplateVersion.ps1
• powershell.exe -NoProfile -File ./scripts/Invoke-RepoChecks.ps1 -IncludeTemplates