fix: stabilize agent file change previews

SovereignAI-Starter/scripts/verify_agent_workspace_live.ps1

@@ -75,11 +75,22 @@ try {
         -ContentType 'application/json' `
         -Body $previewRequest `
         -TimeoutSec 240
+    $previewDiff = [string]$previewResult.proposal.diff
     $unchanged = [IO.File]::ReadAllText($readmePath).Contains('The greet function returns')
-    if ($previewResult.tool -ne 'propose_file_change' -or
-        $previewResult.proposal.path -ne 'README.md' -or
-        -not $unchanged) {
-        throw 'The live agent did not return a README.md diff preview while preserving the file.'
+    if ($previewResult.tool -ne 'propose_file_change') {
+        throw "The live model selected '$($previewResult.tool)' instead of propose_file_change."
+    }
+    if ($previewResult.proposal.path -ne 'README.md') {
+        throw 'The live proposal targeted a different file than README.md.'
+    }
+    if ([string]::IsNullOrWhiteSpace($previewDiff)) {
+        throw 'The live proposal did not include diff text.'
+    }
+    if ($previewDiff -notmatch '(?m)^\+[^+]') {
+        throw 'The live proposal diff did not contain an added line.'
+    }
+    if (-not $unchanged) {
+        throw 'The smoke test changed README.md before explicit approval.'
     }
 
     [pscustomobject]@{
@@ -91,6 +102,7 @@ try {
         preview_tool = $previewResult.tool
         preview_path = $previewResult.proposal.path
         preview_operation = $previewResult.proposal.operation
+        preview_diff_lines = @($previewDiff -split "`n").Count
         file_unchanged_without_user_approval = $unchanged
     } | ConvertTo-Json -Depth 5
 }