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/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);