feat(a2a): 消息重入队列 + 广播丢弃修复 + Router group 管理

A2A 协议完善 (feature-gated by multi-agent):
- AgentInbox wrapper: VecDeque 暂存非匹配消息，requeue 替代丢弃
- a2a_delegate_task: 非匹配消息安全重入队列，不再静默丢弃
- A2aRouter: 广播/组播改用 try_send + 日志，避免持有 RwLock 跨 await
- 新增 group 管理方法: add_to_group/remove_from_group/list_groups/get_group_members
- 修复 Capability import 在 multi-agent feature 下的编译问题

crates/zclaw-protocols/src/a2a.rs

@@ -405,7 +405,15 @@ impl A2aRouter {
                 if let Some(members) = groups.get(group_id) {
                     for agent_id in members {
                         if let Some(tx) = queues.get(agent_id) {
-                            let _ = tx.send(envelope.clone()).await;
+                            match tx.try_send(envelope.clone()) {
+                                Ok(()) => {},
+                                Err(tokio::sync::mpsc::error::TrySendError::Full(_)) => {
+                                    tracing::warn!("A2A delivery to agent {} dropped: channel full", agent_id);
+                                }
+                                Err(tokio::sync::mpsc::error::TrySendError::Closed(_)) => {
+                                    tracing::warn!("A2A delivery to agent {} dropped: channel closed", agent_id);
+                                }
+                            }
                         }
                     }
                 }
@@ -414,7 +422,15 @@ impl A2aRouter {
                 // Broadcast to all registered agents
                 for (agent_id, tx) in queues.iter() {
                     if agent_id != &envelope.from {
-                        let _ = tx.send(envelope.clone()).await;
+                        match tx.try_send(envelope.clone()) {
+                            Ok(()) => {},
+                            Err(tokio::sync::mpsc::error::TrySendError::Full(_)) => {
+                                tracing::warn!("A2A delivery to agent {} dropped: channel full", agent_id);
+                            }
+                            Err(tokio::sync::mpsc::error::TrySendError::Closed(_)) => {
+                                tracing::warn!("A2A delivery to agent {} dropped: channel closed", agent_id);
+                            }
+                        }
                     }
                 }
             }
@@ -444,6 +460,35 @@ impl A2aRouter {
         }
     }
 
+    /// Add agent to a group (creates group if not exists)
+    pub async fn add_to_group(&self, group_id: &str, agent_id: AgentId) {
+        let mut groups = self.groups.write().await;
+        let members = groups.entry(group_id.to_string()).or_insert_with(Vec::new);
+        if !members.contains(&agent_id) {
+            members.push(agent_id);
+        }
+    }
+
+    /// Remove agent from a group
+    pub async fn remove_from_group(&self, group_id: &str, agent_id: &AgentId) {
+        let mut groups = self.groups.write().await;
+        if let Some(members) = groups.get_mut(group_id) {
+            members.retain(|id| id != agent_id);
+        }
+    }
+
+    /// List all groups
+    pub async fn list_groups(&self) -> Vec<String> {
+        let groups = self.groups.read().await;
+        groups.keys().cloned().collect()
+    }
+
+    /// Get members of a group
+    pub async fn get_group_members(&self, group_id: &str) -> Vec<AgentId> {
+        let groups = self.groups.read().await;
+        groups.get(group_id).cloned().unwrap_or_default()
+    }
+
     /// Get all registered agent profiles
     pub async fn list_profiles(&self) -> Vec<A2aAgentProfile> {
         let profiles = self.profiles.read().await;