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;