Add Ctrl+R keybinding for sessions picker in agents window (#315259)

Bind Ctrl+R (Ctrl+R on macOS) to the Show Sessions Picker command,
scoped to the sessions window via IsSessionsWindowContext. This
overrides the Open Recent workspace picker keybinding in the agents
window so Ctrl+R opens the sessions picker instead.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>