ci(windows): free disk space before build (clear prior ISO output)

The boot.wim now carries WinPE-NetFx/PowerShell (collector), growing the image ~0.4GB,
and each build persists a ~5GB ISO to C:\silvermetal\out. On the single-volume runner
that accumulation starved oscdimg ('Insufficient disk space'). Clear prior output +
stale smbuild work dirs at job start so free space self-heals each run.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build-iso-windows.yaml

@@ -40,6 +40,22 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Free disk space
+        shell: pwsh
+        run: |
+          # Each successful build persists a ~5 GB ISO to C:\silvermetal\out, and the
+          # boot.wim now carries WinPE-NetFx/PowerShell (bigger image). On the single-volume
+          # runner that accumulation starves oscdimg ("Insufficient disk space"). Clear the
+          # prior build output + any stale work dir before building so space self-heals.
+          $before = [math]::Round((Get-PSDrive C).Free/1GB,1)
+          Remove-Item 'C:\silvermetal\out\*' -Recurse -Force -ErrorAction SilentlyContinue
+          Get-ChildItem 'C:\gitea-runner\workspace' -Directory -ErrorAction SilentlyContinue | ForEach-Object {
+            $t = Join-Path $_.FullName 'tmp\smbuild'
+            if (Test-Path $t) { Remove-Item $t -Recurse -Force -ErrorAction SilentlyContinue }
+          }
+          $after = [math]::Round((Get-PSDrive C).Free/1GB,1)
+          Write-Host "   C: free ${before}GB -> ${after}GB (cleared prior ISO output + stale smbuild)"
+
       - name: Ensure Windows ADK (oscdimg)
         shell: pwsh
         run: |