ci: remove throwaway runner-probe/runner-prep diagnostics

Their job is done (runner topology mapped, C: extended, ISO staged). The build
+ offline-validation pipeline is green on the runner.

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

.gitea/workflows/runner-prep.yaml

@@ -1,26 +0,0 @@
-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)) }

.gitea/workflows/runner-probe.yaml

@@ -1,27 +0,0 @@
-name: Runner probe (throwaway)
-
-# Temporary diagnostic: discover the silverlabs-runner-win host identity, drives,
-# existing share mounts/stored creds, and whether the base ISO is reachable -
-# so we can pick the simplest path (local path vs persistent SMB mount). Delete
-# once the ISO source is wired. workflow_dispatch only.
-
-on:
-  workflow_dispatch:
-
-jobs:
-  probe:
-    runs-on: windows-latest
-    timeout-minutes: 5
-    steps:
-      - name: Probe SMB as SYSTEM (credential-less net use via stored cmdkey)
-        shell: pwsh
-        env:
-          ISO_SRC: ${{ vars.SILVERMETAL_BASE_ISO_URL }}
-        run: |
-          Write-Host "user=$(whoami)"
-          $root = [regex]::Match($env:ISO_SRC, '^\\\\[^\\]+\\[^\\]+').Value
-          Write-Host "--- credential-less net use $root (should consume stored SYSTEM cmdkey) ---"
-          $r = cmd /c "net use `"$root`"" 2>&1 | Out-String
-          Write-Host $r
-          Write-Host ("UNC exists after net use = {0}" -f [bool](Test-Path -LiteralPath $env:ISO_SRC))
-          cmd /c "net use `"$root`" /delete /y" 2>&1 | Out-Null