fix(terminal): swallow Shift+Enter keypress so it no longer submits

xterm runs the custom key handler for keypress too and drops Ctrl/Alt
keypresses but not Shift-only ones, so the stray \r submitted the prompt
after the newline. Swallow every event type for Shift/Ctrl+Enter and send
only on keydown, in the primary pane and Pane B. Adds a static guard and a
real xterm + Chromium browser test.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

config/test-suites.ts

@@ -31,6 +31,7 @@ export const BROWSER_TEST_GLOBS = [
   'test/capture-geometry-retry.browser.test.ts',
   'test/codex-predictive-echo.test.ts', // also needs a real codex binary
   'test/split-pane-terminal.browser.test.ts',
+  'test/shift-enter-keypress.browser.test.ts',
   'test/split-pane-orchestration.browser.test.ts',
   'test/split-pane-auto-collapse.browser.test.ts',
 ];