feat: publish npm CLI package

[TrickRiggin]

What changed

• add the public agent-guidance-sync npm package metadata and a cross-platform PowerShell CLI shim
• document global npm installation and CLI usage while preserving the repository installer
• validate package metadata against the PowerShell module version
• inspect the npm tarball in Windows and Linux CI

Why

Users can install the tool with one npm command without cloning the repository or mutating their PowerShell module directories. The npm entry point imports the shipped module in place, so the existing preview-first and explicit-apply safety model stays intact.

Validation

• pwsh -NoProfile -File ./tests/Test-AgentGuidanceSync.ps1 - 16 passed
• npm pack --dry-run - 9 intended files, 18.7 kB tarball
• isolated npm install --global --prefix <temp> - CLI shim installed and help executed successfully
• git diff --check