feat(04-01): add Notifier interface and FocusTimer

- Notifier interface with ExecNotifier (notify-send) and NullNotifier
- FocusTimer thread-safe with Set/IsActive/Remaining
- shortName helper for session display names
- Full test coverage for all components

focus.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"sync"
+	"time"
+)
+
+// FocusTimer suppresses notifications for a configurable duration.
+// Thread-safe via mutex.
+type FocusTimer struct {
+	mu      sync.Mutex
+	expires time.Time
+}
+
+// Set activates focus mode for duration d.
+func (f *FocusTimer) Set(d time.Duration) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.expires = time.Now().Add(d)
+}
+
+// IsActive returns true if focus mode has not expired.
+func (f *FocusTimer) IsActive() bool {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return time.Now().Before(f.expires)
+}
+
+// Remaining returns how much focus time is left. Returns 0 if expired.
+func (f *FocusTimer) Remaining() time.Duration {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	rem := time.Until(f.expires)
+	if rem < 0 {
+		return 0
+	}
+	return rem
+}