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/conversation-draft.ts

@@ -5,6 +5,14 @@
  * through `ctx.get` (the inject-free read the app's own plugins use); a
  * missing service or scope degrades to a logged no-op, never a crash.
  *
+ * File references additionally use DSH's own structured insert event
+ * (`slash/input-insert-reference`, see `insertFileReference`) instead of
+ * plain draft text: the native `@file` picker emits this event, and the
+ * conversation input machine mints one occurrence whose chip covers the
+ * whole reference. Plain text `@folder/file.ts` only ever gets DSH's
+ * folder-ref decoration (`@folder/`) — the file name stays undecorated — so
+ * plain append is the fallback, not the primary path, for files.
+ *
  * Insert position (fixes upstream issue #425): the draft store only exposes
  * the whole string (`getSnapshot().draft` + `setDraft(text)`) — there is no
  * caret API on the conversation service. The composer's `<textarea>` keeps
@@ -184,4 +192,64 @@ export function appendToDraft(ctx: Context, sessionId: string, text: string): bo
     console.warn('[dsh-better-sidebar] draft insert failed:', error)
     return false
   }
-}
+}
+
+/**
+ * The DSH `@file` spelling for one relative path, mirroring the host grammar
+ * (`formatFileMention` in `@deepseek-ai/dsh-file-reference`): plain when
+ * there is no whitespace, quoted when there is, and `undefined` when the
+ * path contains a control character or an embedded quote the editor grammar
+ * cannot represent.
+ */
+export function fileMention(relativePath: string): { mention: string; label: string } | undefined {
+  const path = relativePath.replace(/[\\/]+$/, '')
+  if (/[\u0000-\u001f\u007f-\u009f"]/u.test(path)) return undefined
+  const mention = /\s/u.test(path) ? `@"${path}"` : `@${path}`
+  const at = Math.max(path.lastIndexOf('/'), path.lastIndexOf('\\'))
+  const label = at === -1 ? path : path.slice(at + 1)
+  return { mention, label }
+}
+
+/**
+ * Insert one FILE reference as a structured chip (like DSH's own `@` picker).
+ * The chip displays `@<basename>` but serializes to `@<relative path>` on
+ * send, so the reference stays a single link from trigger to basename.
+ *
+ * Directories are NOT handled here: DSH's folder grammar wants the trailing
+ * slash as plain text (`@dir/`) so completion can descend, which
+ * `appendToDraft` already covers.
+ */
+export function insertFileReference(ctx: Context, sessionId: string, relativePath: string): boolean {
+  const reference = fileMention(relativePath)
+  if (reference === undefined) return false
+  try {
+    const actx = ctx.sessions.scope(sessionId)
+    if (actx === undefined) return false
+    const conversation = ctx.get('conversation') as SidebarConversation | undefined
+    if (conversation === undefined) return false
+    const input = conversation.input.for(actx)
+    const before = input.state.getSnapshot()
+    if (before.draftRev === undefined) return false
+    // The session-scope Context's typed `emit` is keyed to DSH's closed event
+    // map; this internal composer event is deliberately string-loose at runtime.
+    ;(actx as unknown as { emit(name: string, payload: unknown): void }).emit('slash/input-insert-reference', {
+      reference: {
+        source: 'reference',
+        ref: reference.mention,
+        label: reference.label,
+        appearance: 'file',
+        clipboardText: reference.mention,
+      },
+      span: {
+        draftRev: before.draftRev,
+        start: before.draft.length,
+        end: before.draft.length,
+      },
+    })
+    const after = input.state.getSnapshot()
+    return after.draftRev !== before.draftRev
+  } catch (error) {
+    console.warn('[dsh-better-sidebar] file-reference insert failed:', error)
+    return false
+  }
+}