feat: add cron tool integration with agent

- Add adhocore/gronx dependency for cron expression parsing
- Fix CronService race conditions and add cron expression support
- Add CronTool with add/list/remove/enable/disable actions
- Add ContextualTool interface for tools needing channel/chatID context
- Add ProcessDirectWithChannel to AgentLoop for cron job execution
- Register CronTool in gateway and wire up onJob handler
- Fix slice bounds panic in addJob for short messages

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

pkg/tools/base.go

@@ -9,6 +9,13 @@ type Tool interface {
 	Execute(ctx context.Context, args map[string]interface{}) (string, error)
 }
 
+// ContextualTool is an optional interface that tools can implement
+// to receive the current message context (channel, chatID)
+type ContextualTool interface {
+	Tool
+	SetContext(channel, chatID string)
+}
+
 func ToolToSchema(tool Tool) map[string]interface{} {
 	return map[string]interface{}{
 		"type": "function",