fix: /clear now clears terminal screen and resets context left indicator (#425)

## What does this PR do?
* Implements the full `/clear` command in **codex‑cli**:
  * Resets chat history **and** wipes the terminal screen.
  * Shows a single system message: `Context cleared`.
* Adds comprehensive unit tests for the new behaviour.

## Why is it needed?
* Fixes user‑reported bugs:  
  * **#395**  
  * **#405**

## How is it implemented?
* **Code** – Adds `process.stdout.write('\x1b[3J\x1b[H\x1b[2J')` in
`terminal.tsx`. Removed reference to `prev` in `
        setItems((prev) => [
          ...prev,
` in `terminal-chat-new-input.tsx` & `terminal-chat-input.tsx`.

## CI / QA
All commands pass locally:
```bash
pnpm test      # green
pnpm run lint  # green
pnpm run typecheck  # zero TS errors
```

## Results



https://github.com/user-attachments/assets/11dcf05c-e054-495a-8ecb-ac6ef21a9da4

---------

Co-authored-by: Thibault Sottiaux <tibo@openai.com>

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

@@ -0,0 +1,174 @@
+import React from "react";
+import type { ComponentProps } from "react";
+import { describe, it, expect, vi } from "vitest";
+import { renderTui } from "./ui-test-helpers.js";
+import TerminalChatInput from "../src/components/chat/terminal-chat-input.js";
+import TerminalChatNewInput from "../src/components/chat/terminal-chat-new-input.js";
+import * as TermUtils from "../src/utils/terminal.js";
+
+// -------------------------------------------------------------------------------------------------
+// Helpers
+// -------------------------------------------------------------------------------------------------
+
+async function type(
+  stdin: NodeJS.WritableStream,
+  text: string,
+  flush: () => Promise<void>,
+): Promise<void> {
+  stdin.write(text);
+  await flush();
+}
+
+// -------------------------------------------------------------------------------------------------
+// Tests
+// -------------------------------------------------------------------------------------------------
+
+describe("/clear command", () => {
+  it("invokes clearTerminal and resets context in TerminalChatInput", async () => {
+    const clearSpy = vi
+      .spyOn(TermUtils, "clearTerminal")
+      .mockImplementation(() => {});
+
+    const setItems = vi.fn();
+
+    // Minimal stub of a ResponseItem – cast to bypass exhaustive type checks in this test context
+    const existingItems = [
+      {
+        id: "dummy-1",
+        type: "message",
+        role: "system",
+        content: [{ type: "input_text", text: "Old item" }],
+      },
+    ] as Array<any>;
+
+    const props: ComponentProps<typeof TerminalChatInput> = {
+      isNew: false,
+      loading: false,
+      submitInput: () => {},
+      confirmationPrompt: null,
+      explanation: undefined,
+      submitConfirmation: () => {},
+      setLastResponseId: () => {},
+      setItems,
+      contextLeftPercent: 100,
+      openOverlay: () => {},
+      openModelOverlay: () => {},
+      openApprovalOverlay: () => {},
+      openHelpOverlay: () => {},
+      openDiffOverlay: () => {},
+      onCompact: () => {},
+      interruptAgent: () => {},
+      active: true,
+      thinkingSeconds: 0,
+      items: existingItems,
+    };
+
+    const { stdin, flush, cleanup } = renderTui(
+      <TerminalChatInput {...props} />,
+    );
+
+    await flush();
+
+    await type(stdin, "/clear", flush);
+    await type(stdin, "\r", flush); // press Enter
+
+    // Allow any asynchronous state updates to propagate
+    await flush();
+
+    expect(clearSpy).toHaveBeenCalledTimes(2);
+    expect(setItems).toHaveBeenCalledTimes(2);
+
+    const stateUpdater = setItems.mock.calls[0]![0];
+    expect(typeof stateUpdater).toBe("function");
+    const newItems = stateUpdater(existingItems);
+    expect(Array.isArray(newItems)).toBe(true);
+    expect(newItems).toHaveLength(2);
+    expect(newItems.at(-1)).toMatchObject({
+      role: "system",
+      type: "message",
+      content: [{ type: "input_text", text: "Terminal cleared" }],
+    });
+
+    cleanup();
+    clearSpy.mockRestore();
+  });
+
+  it("invokes clearTerminal and resets context in TerminalChatNewInput", async () => {
+    const clearSpy = vi
+      .spyOn(TermUtils, "clearTerminal")
+      .mockImplementation(() => {});
+
+    const setItems = vi.fn();
+
+    const props: ComponentProps<typeof TerminalChatNewInput> = {
+      isNew: false,
+      loading: false,
+      submitInput: () => {},
+      confirmationPrompt: null,
+      explanation: undefined,
+      submitConfirmation: () => {},
+      setLastResponseId: () => {},
+      setItems,
+      contextLeftPercent: 100,
+      openOverlay: () => {},
+      openModelOverlay: () => {},
+      openApprovalOverlay: () => {},
+      openHelpOverlay: () => {},
+      openDiffOverlay: () => {},
+      interruptAgent: () => {},
+      active: true,
+      thinkingSeconds: 0,
+    };
+
+    const { stdin, flush, cleanup } = renderTui(
+      <TerminalChatNewInput {...props} />,
+    );
+
+    await flush();
+
+    await type(stdin, "/clear", flush);
+    await type(stdin, "\r", flush); // press Enter
+
+    await flush();
+
+    expect(clearSpy).toHaveBeenCalledTimes(1);
+    expect(setItems).toHaveBeenCalledTimes(1);
+
+    const firstArg = setItems.mock.calls[0]![0];
+    expect(Array.isArray(firstArg)).toBe(true);
+    expect(firstArg).toHaveLength(1);
+    expect(firstArg[0]).toMatchObject({
+      role: "system",
+      type: "message",
+      content: [{ type: "input_text", text: "Terminal cleared" }],
+    });
+
+    cleanup();
+    clearSpy.mockRestore();
+  });
+});
+
+describe("clearTerminal", () => {
+  it("writes escape sequence to stdout", () => {
+    const originalQuiet = process.env["CODEX_QUIET_MODE"];
+    delete process.env["CODEX_QUIET_MODE"];
+
+    process.env["CODEX_QUIET_MODE"] = "0";
+
+    const writeSpy = vi
+      .spyOn(process.stdout, "write")
+      .mockImplementation(() => true);
+
+    TermUtils.clearTerminal();
+
+    expect(writeSpy).toHaveBeenCalledWith("\x1b[3J\x1b[H\x1b[2J");
+
+    writeSpy.mockRestore();
+
+    if (originalQuiet !== undefined) {
+      process.env["CODEX_QUIET_MODE"] = originalQuiet;
+    } else {
+      delete process.env["CODEX_QUIET_MODE"];
+    }
+  });
+});