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/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 && (