Fix crash bugs: remove impossible nil checks, add bounds check for strings.Fields

- mailManager guaranteed non-nil by constructor, removed defensive checks
- Added bounds check for strings.Fields()[0] in slash command validation
- Removed impossible nil check in processMailboxes()

internal/backend/tmux/session.go

@@ -426,7 +426,11 @@ func (s *Session) Send(ctx context.Context, prompt string) (string, error) {
 	// Check command support if applicable
 	if strings.HasPrefix(prompt, "/") && s.commands != nil {
 		if !s.commands.IsSupported(prompt) {
-			cmd := strings.Fields(prompt)[0]
+			fields := strings.Fields(prompt)
+			cmd := "unknown"
+			if len(fields) > 0 {
+				cmd = fields[0]
+			}
 			s.mu.Unlock()
 			return "", fmt.Errorf("slash command not supported by %s backend: %s\nTo use manually, middle-click Attach", s.backendName, cmd)
 		}

internal/session/manager.go

@@ -192,10 +192,6 @@ func (m *Manager) mailProcessingLoop() {
 
 // processMailboxes handles outbox delivery and inbox processing
 func (m *Manager) processMailboxes() {
-	if m.mailManager == nil {
-		return
-	}
-	
 	m.mu.RLock()
 	sessions := make([]backend.Session, 0, len(m.sessions))
 	for _, sess := range m.sessions {
@@ -258,7 +254,7 @@ func (m *Manager) processMailboxes() {
 	}
 }
 
-// GetMailManager returns the mailbox manager
+// GetMailManager returns the mailbox manager (guaranteed non-nil)
 func (m *Manager) GetMailManager() *mailbox.Manager {
 	return m.mailManager
 }