Integrate Obsidian metadataCache across extraction and tooling

Replaces regex-based parsing of frontmatter and headings with
Obsidian's metadataCache where available, falling back to regex
when the cache is unavailable. Propagates App dependency through
constructors to enable cache access.

Key changes:
- ContentExtractor accepts optional cache for frontmatter/headings
- ToolExecutor uses cache for section replacement and frontmatter
- NoteContextBuilder resolves titles/tags from cache
- VaultVectorStore passes cache through indexing pipeline
- AutoTagger checks cache for existing tags instead of content
- Mock updated with metadataCache stubs for tests

src/chat-view.ts

@@ -58,7 +58,7 @@ export class ChatView extends ItemView {
     );
     this.vaultIndexer = new VaultIndexer(this.app.vault, undefined, vectorStore);
     this.toolExecutor = new ToolExecutor(this.app.vault, this.app);
-    this.actionPreviewBuilder = new ActionPreviewBuilder(this.app.vault);
+    this.actionPreviewBuilder = new ActionPreviewBuilder(this.app.vault, this.app);
     this.noteContextBuilder = new NoteContextBuilder(this.app.vault, this.app, this.vaultIndexer);
     this.conversationStateManager = new ConversationStateManager();
     this.workflowEngine = new WorkflowEngine(