feat(04-01): integrate notifications and focus mode into daemon

- Notifier + FocusTimer fields in Daemon, initialized in NewDaemon
- processHookEvent notifies on Working -> Needs Input only (D-01)
- Focus mode suppresses notifications when active (D-05)
- FocusArgs in protocol, "focus" handler in daemon
- CLI "vmux focus <minutes>" command
- Tests for all notification transitions and focus handler

hook.go

@@ -70,8 +70,24 @@ func (d *Daemon) processHookEvent(event HookEvent) {
 		return
 	}
 
+	// Read PrevState BEFORE UpdateFromHook overwrites it
+	d.registry.mu.RLock()
+	prevState := ""
+	if ts, ok := d.registry.sessions[event.SessionID]; ok {
+		prevState = ts.PrevState
+	}
+	d.registry.mu.RUnlock()
+
 	d.registry.UpdateFromHook(event.SessionID, state, waitType, event.Cwd)
 
+	// Notify only on Working -> Needs Input transition (D-01)
+	if state == "Needs Input" && prevState == "Working" && !d.focus.IsActive() {
+		d.registry.mu.RLock()
+		info := d.registry.sessions[event.SessionID].Info
+		d.registry.mu.RUnlock()
+		d.notifier.Notify("vmux: "+shortName(info), "Session needs input ("+waitType+")")
+	}
+
 	d.mu.Lock()
 	d.lastHookTime = time.Now()
 	d.mu.Unlock()