feat(medium): sprint 3 Medium items 6 implementati, 6 confermati gia' presenti

codice:
- Watch-DiskSpace.ps1: Event Log source CI-DiskAlert -> CI-DiskSpaceAlert
  (allineato con task scheduler CI-DiskSpaceAlert)
- Watch-RunnerHealth.ps1: maintenance flag  salta restart se
  F:\CI\State\runner-maintenance.flag esiste
- Invoke-CIJob.ps1:
  - param WebhookUrl (default '') per alert webhook
  - sanity check TemplatePath: Write-Warning se non sotto F:\CI\Templates\
  - 90-min duration warning: Start-Job che posta [WARNING] dopo 5400s
  - finally: Remove-Job per cancellare il warn se build finisce prima

docs:
- BEST-PRACTICES.md sezione 11: VMware HGFS write semantics e cache-poisoning risk
- final-master-plan.md: marcati done  post-failure diagnostics, UseSharedCache,
  VMDK SHA256, SSH host-key, template-refresh runbook, 90-min warn, HGFS,
  Event Log names, TemplatePath sanity, maintenance flag, Setup-Host pass docs,
  emoji-free webhooks

scripts/Watch-DiskSpace.ps1

@@ -69,7 +69,7 @@ $msg = "CI host drive ${DriveLetter}: free space ${freeGB} GB (${pctFree}%) is b
 Write-Warning "[DiskSpace] $msg"
 
 # Windows Event Log
-$logSource = 'CI-DiskAlert'
+$logSource = 'CI-DiskSpaceAlert'
 try {
     if (-not [System.Diagnostics.EventLog]::SourceExists($logSource)) {
         New-EventLog -LogName Application -Source $logSource -ErrorAction Stop