feat: increase vault context limits and make them configurable

- Replace hardcoded 2000-char context limit with configurable maxContextLength
  setting (default: 8000 characters)
- Increase vaultSearchLimit default from 3 to 5 notes
- Add 'Max Context Length' setting to plugin settings UI
- Update README with new defaults
- Update chat-view tests for new setting

src/chat-view.ts

@@ -434,7 +434,7 @@ export class ChatView extends ItemView {
       return;
     }
 
-    const MAX_CONTEXT_LENGTH = 2000;
+    const maxContextLength = this.settings.maxContextLength;
     const tools = this.getTools();
     const messageId = crypto.randomUUID();
     const userMessageId = `${messageId}-user`;
@@ -490,7 +490,7 @@ export class ChatView extends ItemView {
           return parts.join('\n');
         })
         .join('\n\n')
-        .slice(0, MAX_CONTEXT_LENGTH);
+        .slice(0, maxContextLength);
       const userMessageWithContext = context
         ? `Relevant vault context:\n${context}\n\nUser question:\n${userMessage}`
         : userMessage;