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/src/utils/slash-commands.ts

@@ -20,6 +20,7 @@ export const SLASH_COMMANDS: Array<SlashCommand> = [
       "Clear conversation history but keep a summary in context. Optional: /compact [instructions for summarization]",
   },
   { command: "/history", description: "Open command history" },
+  { command: "/sessions", description: "Browse previous sessions" },
   { command: "/help", description: "Show list of commands" },
   { command: "/model", description: "Open model selection panel" },
   { command: "/approval", description: "Open approval mode selection panel" },