feat(01-02): implement signal handling and logging stub

- SignalFlags struct wraps Arc<AtomicBool> for SIGHUP/SIGTERM
- register_signals() registers SIGHUP and SIGTERM only (not SIGINT)
- SIGINT deliberately excluded — handled via crossterm key events for double-press Ctrl+C
- should_terminate() polls flag with Ordering::Relaxed for event loop use
- init_logging() is a Phase 1 no-op stub with LIFE-03 documentation

src/main.rs

@@ -1,4 +1,5 @@
 mod config;
+mod signals;
 mod terminal;
 
 fn main() {