feat(cli): add welcome message and exercise TDD with TTY smoke tests

Verified the TDD workflow by:
1. Adding an expectation for a 'Welcome to Gemini CLI!' message to the TTY bootstrap test.
2. Observing the test failure.
3. Implementing the message in AppHeader.tsx.
4. Verifying the test pass with the built binary.

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

@@ -124,6 +124,14 @@ export const AppHeader = ({ version, showDetails = true }: AppHeaderProps) => {
         />
       )}
 
+      {showHeader && (
+        <Box paddingLeft={2} marginTop={1}>
+          <Text italic color={theme.text.secondary}>
+            Welcome to Gemini CLI!
+          </Text>
+        </Box>
+      )}
+
       {!(settings.merged.ui.hideTips || config.getScreenReader()) &&
         showTips && <Tips config={config} />}
     </Box>