Allow folders in file picker and use them as search filters

- Remove directory filtering from file picker
- When folder selected, insert folder path into input (not as attachment)
- Folder path becomes search query to filter files in that folder
- Files still create attachments as before
- User can navigate folder structure using @ mentions

src/components/file-picker.tsx

@@ -48,14 +48,12 @@ const FilePicker: Component<FilePickerProps> = (props) => {
       console.log(`[FilePicker] Git files response received in ${elapsed}ms:`, gitResponse)
 
       if (gitResponse?.data && gitResponse.data.length > 0) {
-        const gitFiles: FileItem[] = gitResponse.data
-          .filter((file: any) => !file.path.endsWith("/"))
-          .map((file: any) => ({
-            path: file.path,
-            added: file.added,
-            removed: file.removed,
-            isGitFile: true,
-          }))
+        const gitFiles: FileItem[] = gitResponse.data.map((file: any) => ({
+          path: file.path,
+          added: file.added,
+          removed: file.removed,
+          isGitFile: true,
+        }))
         console.log(`[FilePicker] Cached ${gitFiles.length} git files`)
         setCachedGitFiles(gitFiles)
       } else {
@@ -93,23 +91,21 @@ const FilePicker: Component<FilePickerProps> = (props) => {
         console.log(`[FilePicker] Search response received in ${elapsed}ms:`, searchResponse)
 
         searchFiles = (searchResponse?.data || [])
-          .filter((path: string) => !path.endsWith("/"))
           .filter((path: string) => !gitFiles.some((gf) => gf.path === path))
           .map((path: string) => ({
             path,
             isGitFile: false,
           }))
       } else {
-        console.log(`[FilePicker] Empty query, fetching all files with wildcard`)
+        console.log(`[FilePicker] Empty query, fetching all files`)
         const searchResponse = await props.instanceClient.find.files({
-          query: { query: "." },
+          query: { query: "" },
         })
         const elapsed = Date.now() - startTime
 
         console.log(`[FilePicker] All files response received in ${elapsed}ms:`, searchResponse)
 
         searchFiles = (searchResponse?.data || [])
-          .filter((path: string) => !path.endsWith("/"))
           .filter((path: string) => !gitFiles.some((gf) => gf.path === path))
           .map((path: string) => ({
             path,