Add automatic folder creation for file operations and improve chat session handling

Implement `ensureFolderExists` in `ToolExecutor` to create parent directories when creating, moving, or renaming notes.
Update `ChatView` to properly persist agent mode and derive session titles from the first user message. Fix error
handling in tool execution to return structured failures instead of null, and include failure details in follow-up
messages.

__mocks__/obsidian.ts

@@ -5,11 +5,13 @@ export class Vault {
   getMarkdownFiles: () => any[];
   read: (file: any) => Promise<string>;
   create: (path: string, content: string) => Promise<any>;
+  createFolder: (path: string) => Promise<any>;
 
   constructor() {
     this.getMarkdownFiles = () => [];
     this.read = async () => '';
     this.create = async () => null;
+    this.createFolder = async () => null;
   }
 }
 
@@ -113,6 +115,14 @@ export interface TFile {
   path: string;
 }
 
+export class TFolder {
+  path: string;
+
+  constructor(path: string = '') {
+    this.path = path;
+  }
+}
+
 // Plugin class (used by main.ts)
 export class Plugin {
   app: App;