refactor(kernel): 移除 multi-agent feature gate — 33处 cfg 全部删除 (Phase 4A)

8 个文件移除 #[cfg(feature = "multi-agent")]，zclaw-kernel default features
新增 multi-agent。A2A 路由、agents、adapters 现在始终编译。

crates/zclaw-kernel/src/kernel/agents.rs

@@ -2,11 +2,8 @@
 
 use zclaw_types::{AgentConfig, AgentId, AgentInfo, Event, Result};
 
-#[cfg(feature = "multi-agent")]
 use std::sync::Arc;
-#[cfg(feature = "multi-agent")]
 use tokio::sync::Mutex;
-#[cfg(feature = "multi-agent")]
 use super::adapters::AgentInbox;
 
 use super::Kernel;
@@ -23,7 +20,6 @@ impl Kernel {
         self.memory.save_agent(&config).await?;
 
         // Register with A2A router for multi-agent messaging (before config is moved)
-        #[cfg(feature = "multi-agent")]
         {
             let profile = Self::agent_config_to_a2a_profile(&config);
             let rx = self.a2a_router.register_agent(profile).await;
@@ -52,7 +48,6 @@ impl Kernel {
         self.memory.delete_agent(id).await?;
 
         // Unregister from A2A router
-        #[cfg(feature = "multi-agent")]
         {
             self.a2a_router.unregister_agent(id).await;
             self.a2a_inboxes.remove(id);