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/integration-tests/bootstrap.test.ts

@@ -27,8 +27,10 @@ describe('Gemini CLI TTY Bootstrap', () => {
 
     // The ready marker we expect to see
     const readyMarker = 'Type your message or @path/to/file';
+    const welcomeMessage = 'Welcome to Gemini CLI!';
 
-    // Verify the initial render completes and displays the marker
+    // Verify the initial render completes and displays the markers
+    await run.expectText(welcomeMessage, 30000);
     await run.expectText(readyMarker, 30000);
 
     // If we reached here, the smoke test passed

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>