Propagate All CLI Errors to the UI

Priority: P1 Issue: #6146

Remaining Work

• Intercept CLI error output (stderr) in ServerManager / KiloConnectionService
• Show VS Code error notification for critical/startup errors
• Inject error messages into chat UI as system messages for mid-session errors
• Ensure CLI's error SSE events are forwarded to the webview and rendered
• Distinguish between startup errors and runtime errors