Automation rig: add wait-idle or polling helper for scan completion

[techjoec]

Context

The run-all and run-missing automation commands fire-and-forget (return immediately). Scripts must poll get-state and check isBusy to know when scans complete.

Problem

No built-in way to wait for scan completion. Every automation script needs its own polling loop.

Suggested approach

Options (not mutually exclusive):

1. Add a wait-idle command that blocks on the pipe until IsBusy transitions to false (or a timeout)
2. Add a Wait-DriftBusterIdle helper to scripts/vm-automation.ps1 that polls get-state in a loop

Option 2 is simpler and doesn't require changes to the server.

Priority

Medium — scripts work fine with manual polling, but a helper reduces boilerplate.