feat: Enter in the inline prompt textarea submits; Shift/Ctrl+Enter newline

Chat-input convention for the inline prompt's textarea:
  - Enter (no modifier): preventDefault() + click the send button
    (which validates non-empty text and calls onSubmit with the
    current text + the selected provider/model).
  - Shift+Enter / Ctrl+Enter: do NOT preventDefault, so the textarea
    inserts a newline (the default browser behavior).

Implementation:
  - opencode_inline_prompt.ts: add a keydown listener on the textarea
    in the constructor (right after the send button setup, so
    this._sendBtn is already assigned and the closure can call
    this._sendBtn.click()).

Tests:
  - 'Enter in the textarea submits; Shift+Enter does not':
    dispatches keydown Enter on the textarea and asserts the
    onSubmit jest.fn() is called once with the textarea value and the
    current provider/model. Then dispatches Shift+Enter and
    Ctrl+Enter and asserts onSubmit is NOT called.

Verification: pytest 37/37, jest 36/36 (was 35, +1).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/__tests__/opencode_cell_actions.spec.ts

@@ -619,4 +619,59 @@ describe('OpenCodeInlinePrompt', () => {
     const writeText = (navigator.clipboard.writeText as unknown as jest.Mock);
     expect(writeText).toHaveBeenCalledWith('print(1)');
   });
+
+  it('Enter in the textarea submits; Shift+Enter does not', () => {
+    const cell = makeFakeCell('x = 1');
+    const onSubmit = jest.fn();
+    const prompt = new OpenCodeInlinePrompt(cell, {
+      onSubmit,
+      onCancel: jest.fn(),
+      disabled: false,
+      providers: null
+    });
+    prompt.setMessages([
+      { role: 'user', content: '' },
+      { role: 'assistant', content: 'noop' }
+    ]);
+    const textarea = prompt.node.querySelector(
+      'textarea'
+    ) as HTMLTextAreaElement;
+    textarea.value = 'fix the bug';
+
+    // Plain Enter submits with the current text + the current
+    // provider/model selection (both default to "" / first when null).
+    textarea.dispatchEvent(
+      new KeyboardEvent('keydown', {
+        key: 'Enter',
+        bubbles: true,
+        cancelable: true
+      })
+    );
+    expect(onSubmit).toHaveBeenCalledTimes(1);
+    expect(onSubmit).toHaveBeenCalledWith('fix the bug', undefined, undefined);
+
+    // Shift+Enter must NOT submit (chat convention: newline).
+    onSubmit.mockClear();
+    textarea.dispatchEvent(
+      new KeyboardEvent('keydown', {
+        key: 'Enter',
+        shiftKey: true,
+        bubbles: true,
+        cancelable: true
+      })
+    );
+    expect(onSubmit).not.toHaveBeenCalled();
+
+    // Ctrl+Enter also must not submit (newline).
+    onSubmit.mockClear();
+    textarea.dispatchEvent(
+      new KeyboardEvent('keydown', {
+        key: 'Enter',
+        ctrlKey: true,
+        bubbles: true,
+        cancelable: true
+      })
+    );
+    expect(onSubmit).not.toHaveBeenCalled();
+  });
 });