feat(watcher): Phase 2.1 — SessionWatcher goroutine

- internal/watcher: detecte fin de tache via signal file, prompt ❯, idle timeout
- state: ForEachWorking, SetStalled, SetActiveAccount, ActiveAccount
- config: WatcherConfig, DispatcherConfig, JanitorConfig, NotificationsConfig + defaults
- 5 tests unitaires, go test ./... -race OK

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/state/state.go

@@ -133,6 +133,51 @@ func (s *State) SetWorking(name, task string) {
 	s.touch()
 }
 
+// SetStalled marks the named session as stalled (working but heartbeat too old).
+func (s *State) SetStalled(name string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	sess, ok := s.Sessions[name]
+	if !ok {
+		sess = &SessionState{}
+		s.Sessions[name] = sess
+	}
+	sess.State = "stalled"
+	s.touch()
+}
+
+// ForEachWorking calls f for each session currently in "working" state.
+// A snapshot is taken under the read lock; f is called without any lock held.
+func (s *State) ForEachWorking(f func(name string, sess *SessionState)) {
+	s.mu.RLock()
+	working := make(map[string]SessionState, len(s.Sessions))
+	for name, sess := range s.Sessions {
+		if sess.State == "working" {
+			working[name] = *sess
+		}
+	}
+	s.mu.RUnlock()
+	for name, snap := range working {
+		snap := snap
+		f(name, &snap)
+	}
+}
+
+// SetActiveAccount updates the active account in the quota state.
+func (s *State) SetActiveAccount(name string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.Quota.ActiveAccount = name
+	s.touch()
+}
+
+// ActiveAccount returns the current active account name.
+func (s *State) ActiveAccount() string {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.Quota.ActiveAccount
+}
+
 // SetFailed marks the named session as failed and records the failure timestamp.
 // The task is preserved for potential requeue by the caller.
 func (s *State) SetFailed(name string) {