Done-column task cards failed to show file changes when mergeDetails was
missing (e.g. tasks moved to done without the merge flow). The executor
already captures modifiedFiles but TaskCard never read it. Add
modifiedFiles.length as a fallback between mergeDetails.filesChanged and
the worktree-based session files check.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>