feat: add /compact (#289)

Added the ability to compact. Not sure if I should switch the model over
to gpt-4.1 for longer context or if keeping the current model is fine.
Also I'm not sure if setting the compacted to system is best practice,
would love feedback 😄

Mentioned in this issue: https://github.com/openai/codex/issues/230

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

@@ -0,0 +1,31 @@
+import React from "react";
+import type { ComponentProps } from "react";
+import { renderTui } from "./ui-test-helpers.js";
+import TerminalChatInput from "../src/components/chat/terminal-chat-input.js";
+import { describe, it, expect } from "vitest";
+
+describe("TerminalChatInput compact command", () => {
+  it("shows /compact hint when context is low", async () => {
+    const props: ComponentProps<typeof TerminalChatInput> = {
+      isNew: false,
+      loading: false,
+      submitInput: () => {},
+      confirmationPrompt: null,
+      explanation: undefined,
+      submitConfirmation: () => {},
+      setLastResponseId: () => {},
+      setItems: () => {},
+      contextLeftPercent: 10,
+      openOverlay: () => {},
+      openModelOverlay: () => {},
+      openApprovalOverlay: () => {},
+      openHelpOverlay: () => {},
+      onCompact: () => {},
+      interruptAgent: () => {},
+      active: true,
+    };
+    const { lastFrameStripped } = renderTui(<TerminalChatInput {...props} />);
+    const frame = lastFrameStripped();
+    expect(frame).toContain("/compact");
+  });
+});