Format TUI commit activity previews

TESTING.md

@@ -55,6 +55,9 @@ representative repositories.
       double-clicking opens it, and the mouse wheel scrolls the list.
 - [ ] Home activity filters cycle with `v`; activity targets route to their
       repository, issue, pull request, or commit.
+- [ ] A commit activity preview shows its commit count, short hashes, messages,
+      authors, and timestamps as a readable list rather than raw JSON; opening
+      it shows the head commit and its changed files.
 - [ ] Repository favorites toggle with `*`, sort first per pane, and persist
       after relaunch. Commits switch branches with `b`; file trees, text and
       binary files, commit changes, pull changes, and per-file diffs open.