Add keyboard shortcut to create a new workspace

Cmd+N (Ctrl+N off mac) navigates to the new-workspace route through a
global handler keyed on the active host. The sidebar header row surfaces
the shortcut hint on hover, replacing the redundant "+" button that
duplicated the action in the Workspaces section header.

packages/app/src/keyboard/keyboard-action-dispatcher.ts

@@ -26,6 +26,7 @@ export type KeyboardActionId =
   | "workspace.pane.close"
   | "workspace.terminal.new"
   | "sidebar.toggle.right"
+  | "workspace.new"
   | "worktree.new"
   | "worktree.archive";
 
@@ -55,6 +56,7 @@ export type KeyboardActionDefinition =
   | { id: "workspace.pane.close"; scope: KeyboardActionScope }
   | { id: "workspace.terminal.new"; scope: KeyboardActionScope }
   | { id: "sidebar.toggle.right"; scope: KeyboardActionScope }
+  | { id: "workspace.new"; scope: KeyboardActionScope }
   | { id: "worktree.new"; scope: KeyboardActionScope }
   | { id: "worktree.archive"; scope: KeyboardActionScope };