fix: detect remote divergence before sync

sync-skills.ps1

@@ -78,6 +78,15 @@ try {
     }
 
     Write-SyncLog -Level INFO -Message "Starting sync from '$sourceRoot'."
+    Invoke-Git -Arguments @('fetch', 'origin', 'main')
+    $remoteAhead = [int](& git -C $repositoryRoot rev-list --count 'main..origin/main')
+    if ($LASTEXITCODE -ne 0) {
+        throw 'Unable to determine whether the remote branch is ahead.'
+    }
+    if ($remoteAhead -gt 0) {
+        throw "Remote branch origin/main is ahead by $remoteAhead commit(s); merge it before syncing."
+    }
+
     New-Item -ItemType Directory -Path $stagingRoot | Out-Null
 
     $sourceSkills = Get-ChildItem -LiteralPath $sourceRoot -Directory | Sort-Object Name