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

@@ -263,17 +263,16 @@ export default function TerminalChatInput({
         setInput("");
         setSessionId("");
         setLastResponseId("");
+        // Clear the terminal screen (including scrollback) before resetting context
         clearTerminal();
 
-        // Emit a system message to confirm the clear action.  We *append*
-        // it so Ink's <Static> treats it as new output and actually renders it.
-        setItems((prev) => [
-          ...prev,
+        // Print a clear confirmation and reset conversation items.
+        setItems([
           {
             id: `clear-${Date.now()}`,
             type: "message",
             role: "system",
-            content: [{ type: "input_text", text: "Context cleared" }],
+            content: [{ type: "input_text", text: "Terminal cleared" }],
           },
         ]);