Surface CLI Startup Errors in the Extension

Priority: P1 Issue: #6209

StartupErrorBanner exists in webview but VS Code-native notifications for startup failures are missing.

Remaining Work

• Detect CLI process unexpected exit before port announced (process.on('exit', ...) in ServerManager)
• Capture last N lines of stderr
• Show error notification with "Show Logs" and "Retry" buttons
• Add 'failed' state to ConnectionState
• For malformed config: parse error and show human-readable hint