Fix file path format and initialization issues

- Remove leading slash from file paths (use relative paths from workspace)
- Server expects relative paths like 'file.md' not '/file.md'
- Convert isInitialized from let to signal to persist across renders
- Fix file picker not showing files on first open
- Paths from find.files() are already relative to workspace

src/types/attachment.ts

@@ -53,17 +53,16 @@ export function createFileAttachment(
   mime: string = "text/plain",
   data?: Uint8Array,
 ): Attachment {
-  const absolutePath = path.startsWith("/") ? path : `/${path}`
   return {
     id: crypto.randomUUID(),
     type: "file",
     display: `@${filename}`,
-    url: `file://${absolutePath}`,
+    url: path,
     filename,
     mediaType: mime,
     source: {
       type: "file",
-      path: absolutePath,
+      path: path,
       mime,
       data,
     },