feat: Add agent folder structure verification and improve CMake cache cleaning

- Introduced Test-AgentFolderStructure function to verify the presence of required agent files and check for old structure indicators.
- Enhanced Clean-CMakeCache function to provide better error handling and clean additional CMake-generated files.
- Updated build process to include agent folder structure checks before CMake configuration.

src/app/editor/agent/agent_editor.cc

@@ -103,6 +103,9 @@ absl::StatusOr<AgentEditor::SessionInfo> AgentEditor::HostSession(
 
     // Get username from system (could be made configurable)
     const char* username = std::getenv("USER");
+    if (!username) {
+      username = std::getenv("USERNAME");  // Windows fallback
+    }
     if (!username) {
       username = "unknown";
     }
@@ -172,6 +175,9 @@ absl::StatusOr<AgentEditor::SessionInfo> AgentEditor::JoinSession(
     }
 
     const char* username = std::getenv("USER");
+    if (!username) {
+      username = std::getenv("USERNAME");  // Windows fallback
+    }
     if (!username) {
       username = "unknown";
     }