ci: runner-prep workflow (extend C: only); drop in-CI ISO staging

Master creds must not live in this public repo's Actions, so ISO staging is
handled out-of-band. runner-prep now only extends C: into the resized virtual
disk. Quoted the step name (trailing-colon YAML fix).

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

.gitea/workflows/runner-prep.yaml

@@ -0,0 +1,26 @@
+name: Runner prep (throwaway)
+
+# One-time prep on silverlabs-runner-win (GITEA-RUN-WIN, SYSTEM): extend C: into
+# the newly-added virtual-disk space (Proxmox resize already done host-side).
+# ISO staging is handled separately (creds must not live in this public repo's CI).
+# Delete this workflow once prep is done. workflow_dispatch only.
+
+on:
+  workflow_dispatch:
+
+jobs:
+  prep:
+    runs-on: windows-latest
+    timeout-minutes: 15
+    steps:
+      - name: "Extend C: into added space"
+        shell: pwsh
+        run: |
+          'rescan' | diskpart | Out-Null
+          $max = (Get-PartitionSupportedSize -DriveLetter C).SizeMax
+          $cur = (Get-Partition -DriveLetter C).Size
+          if ($cur -lt ($max - 1GB)) {
+            Write-Host ("Extending C: {0} GB -> {1} GB" -f [math]::Round($cur/1GB,1), [math]::Round($max/1GB,1))
+            Resize-Partition -DriveLetter C -Size $max
+          } else { Write-Host 'C: already at max; nothing to extend.' }
+          Get-PSDrive C | ForEach-Object { Write-Host ("C: free now {0} GB" -f [math]::Round($_.Free/1GB,1)) }