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,

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");

codex-cli/tests/terminal-chat-input-compact.test.tsx

@@ -21,6 +21,7 @@ describe("TerminalChatInput compact command", () => {
       openModelOverlay: () => {},
       openApprovalOverlay: () => {},
       openHelpOverlay: () => {},
+      openSessionsOverlay: () => {},
       onCompact: () => {},
       interruptAgent: () => {},
       active: true,

codex-cli/tests/terminal-chat-input-file-tag-suggestions.test.tsx

@@ -76,6 +76,7 @@ describe("TerminalChatInput file tag suggestions", () => {
     openModelOverlay: vi.fn(),
     openApprovalOverlay: vi.fn(),
     openHelpOverlay: vi.fn(),
+    openSessionsOverlay: vi.fn(),
     onCompact: vi.fn(),
     interruptAgent: vi.fn(),
     active: true,

codex-cli/tests/terminal-chat-input-multiline.test.tsx

@@ -42,6 +42,7 @@ describe("TerminalChatInput multiline functionality", () => {
       openModelOverlay: () => {},
       openApprovalOverlay: () => {},
       openHelpOverlay: () => {},
+      openSessionsOverlay: () => {},
       onCompact: () => {},
       interruptAgent: () => {},
       active: true,
@@ -93,6 +94,7 @@ describe("TerminalChatInput multiline functionality", () => {
       openModelOverlay: () => {},
       openApprovalOverlay: () => {},
       openHelpOverlay: () => {},
+      openSessionsOverlay: () => {},
       onCompact: () => {},
       interruptAgent: () => {},
       active: true,