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/service.ts

@@ -151,7 +151,7 @@ export interface TabComponentProps {
   /** The explorer's reveal-highlight set (ExplorerView; "Show in folder" targets). */
   revealed?: string[]
   onToggleDir?: (path: string) => void
-  onReferenceFile?: (path: string) => void
+  onReferenceFile?: (path: string, isDir: boolean) => void
   onOpenFile?: (path: string) => void
   onOpenDiff?: (tab: SidebarTab) => void
   onSubagentJump?: (childSessionId: string) => void