Add the _time_travel core plugin with Agent Zero-owned shadow Git snapshots, history/diff/preview/travel/revert APIs, capture hooks, and canvas plus floating window UI surfaces for /a0/usr workspaces.
Wire generic file-browser mutation hooks for UI edits, update modal backdrop handling, remove the legacy _diff_viewer plugin, and replace Diff Viewer tests with focused Time Travel coverage.
Inspired by Space Agent :-)
