fix(tui): add Ctrl+J as newline fallback in chat input

Terminals without extended key reporting (VS Code, Cursor) send
identical events for Enter and Shift+Enter, making it impossible
to insert newlines. Ctrl+J produces a distinct key event in all
terminals.

core/framework/tui/widgets/chat_repl.py

@@ -33,7 +33,7 @@ from framework.tui.widgets.selectable_rich_log import SelectableRichLog as RichL
 
 
 class ChatTextArea(TextArea):
-    """TextArea that submits on Enter and inserts newlines on Shift+Enter."""
+    """TextArea that submits on Enter and inserts newlines on Shift+Enter (or Ctrl+J)."""
 
     class Submitted(Message):
         """Posted when the user presses Enter."""
@@ -50,7 +50,7 @@ class ChatTextArea(TextArea):
                 self.post_message(self.Submitted(text))
             event.stop()
             event.prevent_default()
-        elif event.key == "shift+enter":
+        elif event.key in ("shift+enter", "ctrl+j"):
             event.key = "enter"
             await super()._on_key(event)
         else:

core/tests/test_chat_text_area.py

@@ -0,0 +1,90 @@
+"""Tests for ChatTextArea key handling (Enter submits, Shift+Enter / Ctrl+J insert newlines)."""
+
+import pytest
+from textual.app import App, ComposeResult
+
+from framework.tui.widgets.chat_repl import ChatTextArea
+
+
+class ChatTextAreaApp(App):
+    """Minimal app that mounts a ChatTextArea for testing."""
+
+    submitted_texts: list[str]
+
+    def compose(self) -> ComposeResult:
+        yield ChatTextArea(id="input")
+
+    def on_mount(self) -> None:
+        self.submitted_texts = []
+
+    def on_chat_text_area_submitted(self, message: ChatTextArea.Submitted) -> None:
+        self.submitted_texts.append(message.text)
+
+
+@pytest.fixture
+def app():
+    return ChatTextAreaApp()
+
+
+@pytest.mark.asyncio
+async def test_enter_submits_text(app):
+    """Pressing Enter should post a Submitted message and clear the widget."""
+    async with app.run_test() as pilot:
+        await pilot.press("h", "e", "l", "l", "o")
+        await pilot.press("enter")
+
+    assert app.submitted_texts == ["hello"]
+
+
+@pytest.mark.asyncio
+async def test_enter_on_empty_does_not_submit(app):
+    """Pressing Enter with no text should not post a Submitted message."""
+    async with app.run_test() as pilot:
+        await pilot.press("enter")
+
+    assert app.submitted_texts == []
+
+
+@pytest.mark.asyncio
+async def test_shift_enter_inserts_newline(app):
+    """Shift+Enter should insert a newline, not submit."""
+    async with app.run_test() as pilot:
+        widget = app.query_one("#input", ChatTextArea)
+
+        await pilot.press("a")
+        await pilot.press("shift+enter")
+        await pilot.press("b")
+
+    assert app.submitted_texts == []
+    assert "\n" in widget.text
+    assert widget.text.startswith("a")
+    assert widget.text.endswith("b")
+
+
+@pytest.mark.asyncio
+async def test_ctrl_j_inserts_newline(app):
+    """Ctrl+J should insert a newline (fallback for terminals without Shift+Enter)."""
+    async with app.run_test() as pilot:
+        widget = app.query_one("#input", ChatTextArea)
+
+        await pilot.press("a")
+        await pilot.press("ctrl+j")
+        await pilot.press("b")
+
+    assert app.submitted_texts == []
+    assert "\n" in widget.text
+    assert widget.text.startswith("a")
+    assert widget.text.endswith("b")
+
+
+@pytest.mark.asyncio
+async def test_multiline_submit(app):
+    """Typing multiline text via Ctrl+J then pressing Enter should submit all lines."""
+    async with app.run_test() as pilot:
+        await pilot.press("a")
+        await pilot.press("ctrl+j")
+        await pilot.press("b")
+        await pilot.press("enter")
+
+    assert len(app.submitted_texts) == 1
+    assert app.submitted_texts[0] == "a\nb"