feat(tui): add input history for submitted messages (WIP)

Record submitted inputs in the editor history so up/down arrow
can recall previous messages.

Adds a small helper to wire submit handling and unit tests for
routing/recording behavior.

No PR yet (per request).

src/tui/tui-input-history.test.ts

@@ -0,0 +1,99 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { createEditorSubmitHandler } from "./tui.js";
+
+describe("createEditorSubmitHandler", () => {
+  it("adds submitted messages to editor history", () => {
+    const editor = {
+      setText: vi.fn(),
+      addToHistory: vi.fn(),
+    };
+
+    const handler = createEditorSubmitHandler({
+      editor,
+      handleCommand: vi.fn(),
+      sendMessage: vi.fn(),
+    });
+
+    handler("hello world");
+
+    expect(editor.setText).toHaveBeenCalledWith("");
+    expect(editor.addToHistory).toHaveBeenCalledWith("hello world");
+  });
+
+  it("trims input before adding to history", () => {
+    const editor = {
+      setText: vi.fn(),
+      addToHistory: vi.fn(),
+    };
+
+    const handler = createEditorSubmitHandler({
+      editor,
+      handleCommand: vi.fn(),
+      sendMessage: vi.fn(),
+    });
+
+    handler("   hi   ");
+
+    expect(editor.addToHistory).toHaveBeenCalledWith("hi");
+  });
+
+  it("does not add empty submissions to history", () => {
+    const editor = {
+      setText: vi.fn(),
+      addToHistory: vi.fn(),
+    };
+
+    const handler = createEditorSubmitHandler({
+      editor,
+      handleCommand: vi.fn(),
+      sendMessage: vi.fn(),
+    });
+
+    handler("   ");
+
+    expect(editor.addToHistory).not.toHaveBeenCalled();
+  });
+
+  it("routes slash commands to handleCommand", () => {
+    const editor = {
+      setText: vi.fn(),
+      addToHistory: vi.fn(),
+    };
+    const handleCommand = vi.fn();
+    const sendMessage = vi.fn();
+
+    const handler = createEditorSubmitHandler({
+      editor,
+      handleCommand,
+      sendMessage,
+    });
+
+    handler("/models");
+
+    expect(editor.addToHistory).toHaveBeenCalledWith("/models");
+    expect(handleCommand).toHaveBeenCalledWith("/models");
+    expect(sendMessage).not.toHaveBeenCalled();
+  });
+
+  it("routes normal messages to sendMessage", () => {
+    const editor = {
+      setText: vi.fn(),
+      addToHistory: vi.fn(),
+    };
+    const handleCommand = vi.fn();
+    const sendMessage = vi.fn();
+
+    const handler = createEditorSubmitHandler({
+      editor,
+      handleCommand,
+      sendMessage,
+    });
+
+    handler("hello");
+
+    expect(editor.addToHistory).toHaveBeenCalledWith("hello");
+    expect(sendMessage).toHaveBeenCalledWith("hello");
+    expect(handleCommand).not.toHaveBeenCalled();
+  });
+});