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/slash-commands.test.ts

@@ -6,6 +6,7 @@ test("SLASH_COMMANDS includes expected commands", () => {
   expect(commands).toContain("/clear");
   expect(commands).toContain("/compact");
   expect(commands).toContain("/history");
+  expect(commands).toContain("/sessions");
   expect(commands).toContain("/help");
   expect(commands).toContain("/model");
   expect(commands).toContain("/approval");