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

tests/chat-view.test.ts

@@ -31,6 +31,7 @@ const mockSettings: PluginSettings = {
   model: 'llama3',
   vaultSearchLimit: 3,
   maxMessageHistory: 50,
+  maxContextLength: 8000,
   lastIndexTime: 0,
   cacheConfig: {
     enabled: false,