fix(ci): stabilize sync-guard runtime list handling

[svelderrainruiz]

Summary

• replace generic list mutation paths with native arrays in Test-ForkUpstreamSyncGuard.ps1
• remove PowerShell binder failure (Argument types do not match) when building drift report payload

Validation

• Invoke-Pester -Path ./tests -CI (20/20 passed)
• manual script invocation now emits report and fails with deterministic reason (main_head) when drift exists