ci: add project validation workflow

[Victorzinn704]

Summary

• adds a GitHub Actions CI workflow for Project Sentinel
• validates Go dependency download, tests and build using the Go version from go.mod
• runs the existing PowerShell publication guard on Windows
• uses checkout v5 and setup-go v6 to avoid Node 20 action runtime warnings

Local validation

• powershell -NoProfile -ExecutionPolicy Bypass -File .\tools\publication_guard.ps1
• git diff --check

Notes

• Go is not installed in the current local Windows environment, so Go test/build validation is intentionally delegated to GitHub Actions.