fix(acp): relativize the completed diff card title

The result-time diff card sent view.title raw, so a completed edit/write
of an absolute in-workspace path flipped the card header back from the
relativized `Edit src/b.ts` to the absolute path — the pending card
relativizes, the result did not, and tool_call_update.title replaces the
header. Apply displayTitle to the result diff arm using the diff path,
mirroring the call-side card. Regression test proven red on the unfixed
arm.

Also record the overwrite diff-basis pre-read as a bounded follow-up
(TODO(overwrite-diff-bound) + RFC non-goal): overwriting a large file
reads the whole prior text into memory for a UI-only diff.

packages/ui/acp/src/index.ts

@@ -1178,12 +1178,17 @@ function toolResultUpdate(callId: CallId, view: ToolResultView, isError: boolean
       // the diff the pending card installed (and keeps the model-facing result
       // text from clobbering it).
       const content: AcpToolCallContent[] = view.diffs.map(d => ({ type: 'diff', path: d.path, oldText: d.oldText, newText: d.newText }))
+      // Relativize the replacement title against the session cwd from the diff
+      // path, exactly as the call-side card does — `tool_call_update.title`
+      // replaces the card header, so a raw absolute path here would undo the
+      // pending card's relativized title.
+      const title = view.title !== undefined ? displayTitle(view.title, view.diffs[0]?.path, terminal.cwd) : undefined
       return {
         sessionUpdate: 'tool_call_update',
         toolCallId: callId,
         status,
         ...content.length > 0 ? { content } : {},
-        ...view.title !== undefined ? { title: view.title } : {},
+        ...title !== undefined ? { title } : {},
       }
     }
     default: