fix(editor): keep file basename in @-reference links

The explorer @ button appended plain text (@folder/file.ts), which DSH's
composer only decorates as a folder ref (@folder/) — the file name stayed
outside the link. Insert FILE references through DSH's own structured
insert-reference event instead, minting one chip for the whole reference
(displayed as @basename, serialized as the full @path). Directories keep the
plain @dir/ mention so folder completion still works.

src/client/EditorHost.tsx

@@ -100,7 +100,7 @@ export function EditorHost(props: {
   expanded: string[]
   revealed: string[]
   onToggleDir: (path: string) => void
-  onReferenceFile: (path: string) => void
+  onReferenceFile: (path: string, isDir: boolean) => void
 }) {
   const { ctx, store, scope, tab, expanded, revealed, onToggleDir, onReferenceFile } = props
   const path = tab.path ?? ''