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/components/chat/terminal-chat-input.tsx

@@ -54,6 +54,7 @@ export default function TerminalChatInput({
   openApprovalOverlay,
   openHelpOverlay,
   openDiffOverlay,
+  openSessionsOverlay,
   onCompact,
   interruptAgent,
   active,
@@ -77,6 +78,7 @@ export default function TerminalChatInput({
   openApprovalOverlay: () => void;
   openHelpOverlay: () => void;
   openDiffOverlay: () => void;
+  openSessionsOverlay: () => void;
   onCompact: () => void;
   interruptAgent: () => void;
   active: boolean;
@@ -280,6 +282,9 @@ export default function TerminalChatInput({
                 case "/history":
                   openOverlay();
                   break;
+                case "/sessions":
+                  openSessionsOverlay();
+                  break;
                 case "/help":
                   openHelpOverlay();
                   break;
@@ -484,6 +489,10 @@ export default function TerminalChatInput({
         setInput("");
         openOverlay();
         return;
+      } else if (inputValue === "/sessions") {
+        setInput("");
+        openSessionsOverlay();
+        return;
       } else if (inputValue === "/help") {
         setInput("");
         openHelpOverlay();
@@ -728,6 +737,7 @@ export default function TerminalChatInput({
       openModelOverlay,
       openHelpOverlay,
       openDiffOverlay,
+      openSessionsOverlay,
       history,
       onCompact,
       skipNextSubmit,