fix: use merge-base for diffStat in merger and show files changed on done task cards

The merger was using `git diff HEAD..branch --stat` which includes
artifacts from other tasks when branches fork from older main commits.
Switch to `git diff $(merge-base)..branch --stat` so commit messages
only describe the branch's own changes.

Also surface the "files changed" button on done task cards using
mergeDetails, opening the same ChangedFilesModal with commit-backed
diffs (matching the Changes tab in the task modal).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/Board.tsx

@@ -35,7 +35,7 @@ interface BoardProps {
    * Called when the user clicks the "Subtask" button in the inline create card.
    */
   onSubtaskBreakdown?: (description: string) => void;
-  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string) => void;
+  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string, commitSha?: string) => void;
   favoriteProviders?: string[];
   favoriteModels?: string[];
   onToggleFavorite?: (provider: string) => void;

packages/dashboard/app/components/ChangedFilesModal.tsx

@@ -19,6 +19,7 @@ interface ChangedFilesModalProps {
   worktree: string | undefined;
   column: string;
   projectId?: string;
+  commitSha?: string;
   isOpen: boolean;
   onClose: () => void;
 }
@@ -66,6 +67,7 @@ export function ChangedFilesModal({
   worktree,
   column,
   projectId,
+  commitSha,
   isOpen,
   onClose,
 }: ChangedFilesModalProps) {
@@ -74,6 +76,7 @@ export function ChangedFilesModal({
     worktree,
     column,
     projectId,
+    commitSha,
   );
 
   const [isMobile, setIsMobile] = useState(false);

packages/dashboard/app/components/Column.tsx

@@ -46,7 +46,7 @@ interface ColumnProps {
    * Called when the user clicks the "Subtask" button in the inline create card.
    */
   onSubtaskBreakdown?: (description: string) => void;
-  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string) => void;
+  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string, commitSha?: string) => void;
   favoriteProviders?: string[];
   favoriteModels?: string[];
   onToggleFavorite?: (provider: string) => void;

packages/dashboard/app/components/TaskCard.tsx

@@ -44,7 +44,7 @@ interface TaskCardProps {
   ) => Promise<Task>;
   onArchiveTask?: (id: string) => Promise<Task>;
   onUnarchiveTask?: (id: string) => Promise<Task>;
-  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string) => void;
+  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string, commitSha?: string) => void;
 }
 
 function areTaskBadgeInfosEqual(
@@ -688,6 +688,20 @@ function TaskCardComponent({
           </span>
         </button>
       )}
+      {task.column === "done" && task.mergeDetails?.filesChanged != null && task.mergeDetails.filesChanged > 0 && (
+        <button
+          type="button"
+          className="card-session-files"
+          onClick={(e) => {
+            e.stopPropagation();
+            onOpenFilesForTask?.(task.id, task.worktree, task.column, task.mergeDetails?.commitSha);
+          }}
+          disabled={!onOpenFilesForTask}
+        >
+          <Folder size={12} />
+          <span>{task.mergeDetails.filesChanged} files changed</span>
+        </button>
+      )}
       {((task.dependencies && task.dependencies.length > 0) || queued || task.status === "queued" || task.blockedBy) && (
         <div className="card-meta">
           {task.dependencies && task.dependencies.length > 0 && (

packages/dashboard/app/components/WorktreeGroup.tsx

@@ -16,7 +16,7 @@ interface WorktreeGroupProps {
     id: string,
     updates: { title?: string; description?: string; dependencies?: string[] }
   ) => Promise<Task>;
-  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string) => void;
+  onOpenFilesForTask?: (taskId: string, worktree: string | undefined, column: string, commitSha?: string) => void;
 }
 
 function WorktreeGroupComponent({