fix: eliminate VM004, VM005, VM006 violations in MainWindowViewModel

Replace direct Process.Start calls with _processLauncher service methods.
Replace Timer/FileSystemWatcher with _timerService/_serviceFactory abstractions.
Replace JsonDocument.Parse/JsonNode.Parse with _jsonParser service methods.
Move WatchFileSystem factory method to McpServiceFactory to avoid
FileSystemWatcher type name in ViewModel file.
Remove dead code: OnAgentsReadmeFileChanged, OnAgentsReadmeFileRenamed,
OnAgentsReadmeWatcherError event handlers (replaced by watcher callback).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sharpninja

src/McpServerManager.Core/Services/McpServiceFactory.cs

@@ -1,5 +1,6 @@
 using System;
 using McpServer.Client;
+using McpServer.UI.Core.Services;
 
 namespace McpServerManager.Core.Services;
 
@@ -72,4 +73,10 @@ public McpAgentEventStreamService CreateEventStreamService(
             resolveApiKey: resolveApiKey,
             resolveWorkspacePath: resolveWorkspacePath);
     }
+
+    /// <summary>
+    /// Watches a directory for file changes matching the given filter.
+    /// </summary>
+    public IWatcherHandle WatchFileSystem(string directory, string filter, Action<string> onChanged)
+        => new Infrastructure.FileSystemWatcherService().Watch(directory, filter, onChanged);
 }