add: session history viewer (#912)

- A new “/sessions” command is available for browsing previous sessions,
as shown in the updated slash command list

- The CLI now documents and parses a new “--history” flag to browse past
sessions from the command line

- A dedicated `SessionsOverlay` component loads session metadata and
allows toggling between viewing and resuming sessions

- When the sessions overlay is opened during a chat, selecting a session
can either show the saved rollout or resume it

codex-cli/tests/clear-command.test.tsx

@@ -55,6 +55,7 @@ describe("/clear command", () => {
       openApprovalOverlay: () => {},
       openHelpOverlay: () => {},
       openDiffOverlay: () => {},
+      openSessionsOverlay: () => {},
       onCompact: () => {},
       interruptAgent: () => {},
       active: true,