Implement client-side file scanning with gitignore support

- Use Node.js fs APIs to recursively scan workspace folder
- Load and parse .gitignore to filter files
- Cache scanned files for performance
- Filter files by search query on client side
- Skip .git and node_modules directories
- Support gitignore patterns (basic wildcards)
- No server API calls needed for file listing

src/components/prompt-input.tsx

@@ -450,6 +450,7 @@ export default function PromptInput(props: PromptInputProps) {
             instanceClient={instance()!.client}
             searchQuery={fileSearchQuery()}
             textareaRef={textareaRef}
+            workspaceFolder={props.instanceFolder}
           />
         </Show>