fix: unify diff endpoints and show changed files on done task cards

- Server /tasks/:id/diff and /tasks/:id/file-diffs now both use
  resolveDiffBase() for consistent file lists across card and modal
- Both endpoints handle done tasks via mergeDetails.commitSha,
  returning structured { files, stats } from the merge commit
- TaskChangesTab uses fetchTaskDiff for all columns (no more
  fetchCommitDiff/parsePatch client-side path for done tasks)
- useChangedFiles hook simplified — server handles done tasks
- Done task cards show "N files changed" button using mergeDetails
- Updated tests for new unified data flow

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

packages/dashboard/app/components/ChangedFilesModal.tsx

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