Add Git worktree pane

docs/USER_GUIDE.md

@@ -20,6 +20,14 @@ the current local branch. An unsaved draft can be moved with the project menu;
 saved chats keep their original project. Use the branch menu to switch local
 branches when that project's chats are idle.
 
+The **Git** pane shows added, changed, and deleted files from the current
+worktree, including separate staged and worktree markers. Select files to stage,
+unstage, or commit them, or commit the existing staged set. Committing with no
+selection or staged files asks before staging and committing every change. File
+names open a colored diff that can switch between unified and side-by-side
+layouts. The fetch, pull, and push actions use the `origin` remote and report
+authentication, hook, conflict, and other Git errors without hiding them.
+
 Quitting with active chats asks for confirmation. Confirming stops their model
 and tool work; canceling leaves every chat running.