Fix handling of Shift+Enter in e.g. Ghostty (#338)

Fix: Shift + Enter no longer prints “[27;2;13~” in the single‑line
input. Validated as working and necessary in Ghostty on Linux.

## Key points
- src/components/vendor/ink-text-input.tsx
- Added early handler that recognises the two modifyOtherKeys
escape‑sequences
    - [13;<mod>u  (mode 2 / CSI‑u)
    - [27;<mod>;13~ (mode 1 / legacy CSI‑~)
- If Ctrl is held (hasCtrl flag) → call onSubmit() (same as plain
Enter).
- Otherwise → insert a real newline at the caret (same as Option+Enter).
  - Prevents the raw sequence from being inserted into the buffer.

- src/components/chat/multiline-editor.tsx
- Replaced non‑breaking spaces with normal spaces to satisfy eslint
no‑irregular‑whitespace rule (no behaviour change).

All unit tests (114) and ESLint now pass:
npm test ✔️
npm run lint ✔️

codex-cli/tests/multiline-shift-enter-mod1.test.tsx

@@ -0,0 +1,49 @@
+// Regression test: Terminals with modifyOtherKeys=1 emit CSI~ sequence for
+// Shift+Enter: ESC [ 27 ; mod ; 13 ~.  The editor must treat Shift+Enter as
+// newline (without submitting) and Ctrl+Enter as submit.
+
+import { renderTui } from "./ui-test-helpers.js";
+import MultilineTextEditor from "../src/components/chat/multiline-editor.js";
+import * as React from "react";
+import { describe, it, expect, vi } from "vitest";
+
+async function type(
+  stdin: NodeJS.WritableStream,
+  text: string,
+  flush: () => Promise<void>,
+) {
+  stdin.write(text);
+  await flush();
+}
+
+describe("MultilineTextEditor – Shift+Enter with modifyOtherKeys=1", () => {
+  it("inserts newline, does NOT submit", async () => {
+    const onSubmit = vi.fn();
+
+    const { stdin, lastFrameStripped, flush, cleanup } = renderTui(
+      React.createElement(MultilineTextEditor, {
+        height: 5,
+        width: 20,
+        initialText: "",
+        onSubmit,
+      }),
+    );
+
+    await flush();
+
+    await type(stdin, "abc", flush);
+    // Shift+Enter => ESC [27;2;13~
+    await type(stdin, "\u001B[27;2;13~", flush);
+    await type(stdin, "def", flush);
+
+    const frame = lastFrameStripped();
+    expect(frame).toMatch(/abc/);
+    expect(frame).toMatch(/def/);
+    // newline inserted -> at least 2 lines
+    expect(frame.split("\n").length).toBeGreaterThanOrEqual(2);
+
+    expect(onSubmit).not.toHaveBeenCalled();
+
+    cleanup();
+  });
+});