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/TreePanel.tsx

@@ -62,7 +62,7 @@ export function TreePanel(props: {
   openWithSsh?: boolean
   onOpenWith?: (targetId: string, path: string) => void
   onToggleOpenWithPin?: (targetId: string) => void
-  onReferenceFile: (path: string) => void
+  onReferenceFile: (path: string, isDir: boolean) => void
   /** Full-window presentation: the panel fills its host instead of docking
    *  at a fixed width. */
   full?: boolean