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/route-shortcut.test.ts

@@ -29,6 +29,7 @@ describe("routeKeyboardShortcut — dispatch passthroughs", () => {
   it.each([
     ["agent.interrupt", { id: "agent.interrupt", scope: "global" }],
     ["workspace.tab.new", { id: "workspace.tab.new", scope: "workspace" }],
+    ["workspace.new", { id: "workspace.new", scope: "sidebar" }],
     ["worktree.archive", { id: "worktree.archive", scope: "sidebar" }],
     ["worktree.new", { id: "worktree.new", scope: "sidebar" }],
     ["workspace.terminal.new", { id: "workspace.terminal.new", scope: "workspace" }],