fix: prevent Shift+Tab from accepting prompt placeholder suggestion (#3051)

Add !key.shift guard to the Tab key handler for prompt suggestion
acceptance, so Shift+Tab only toggles approval mode without inserting
the placeholder text into the input.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/cli/src/ui/components/InputPrompt.tsx

@@ -719,6 +719,7 @@ export const InputPrompt: React.FC<InputPromptProps> = ({
       // because ACCEPT_SUGGESTION also matches Enter which must fall through to SUBMIT.
       if (
         key.name === 'tab' &&
+        !key.shift &&
         buffer.text.length === 0 &&
         !completion.showSuggestions &&
         !reverseSearchActive &&