feat(agent-manager): add keyboard shortcuts dialog to settings menu (#524)

Add a Keyboard Shortcuts item to the settings gear dropdown in the
Agent Manager sidebar. Opens a kilo-ui Dialog showing all available
shortcuts grouped by category (Sidebar, Tabs, Terminal, Global) with
platform-appropriate key rendering (Mac symbols / Windows key names).

Also exposes the global agentManagerOpen keybinding so it appears in
the dialog alongside the agent-manager-scoped shortcuts.