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/actions.ts

@@ -41,6 +41,7 @@ export type KeyboardActionId =
   | "command-center.toggle"
   | "shortcuts.dialog.toggle"
   | "workspace.terminal.new"
+  | "workspace.new"
   | "worktree.new"
   | "worktree.archive"
   | "view.toggle.focus"