Make agent file writes append by default

MeetingAssistant/Workflow/WorkflowRulesEditorTools.cs

@@ -51,7 +51,7 @@ public sealed class WorkflowRulesEditorTools
         return ReadLines(await File.ReadAllTextAsync(rulesPath), from, to);
     }
 
-    public async Task<string> WriteRules(string yaml)
+    public async Task<string> WriteRules(string yaml, bool replace_file = false)
     {
         if (rulesPath is null)
         {
@@ -63,14 +63,20 @@ public sealed class WorkflowRulesEditorTools
             return "Refused: yaml must not be null.";
         }
 
-        var validation = ValidateYaml(yaml);
+        var existing = File.Exists(rulesPath)
+            ? await File.ReadAllTextAsync(rulesPath)
+            : "";
+        var updated = replace_file
+            ? yaml
+            : AppendContent(existing, yaml);
+        var validation = ValidateYaml(updated);
         if (validation is not null)
         {
             return validation;
         }
 
         Directory.CreateDirectory(Path.GetDirectoryName(rulesPath)!);
-        await File.WriteAllTextAsync(rulesPath, yaml);
+        await File.WriteAllTextAsync(rulesPath, updated);
         return rulesPath;
     }
 
@@ -359,6 +365,24 @@ public sealed class WorkflowRulesEditorTools
         }
     }
 
+    private static string AppendContent(string existingContent, string content)
+    {
+        if (string.IsNullOrEmpty(existingContent))
+        {
+            return content;
+        }
+
+        if (string.IsNullOrEmpty(content))
+        {
+            return existingContent;
+        }
+
+        var separator = existingContent.EndsWith('\n') || existingContent.EndsWith('\r')
+            ? ""
+            : "\n";
+        return existingContent + separator + content;
+    }
+
     private static async Task<string?> RunRipgrepAsync(string rulesPath, string keywords)
     {
         try