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/mod.rs

@@ -8,16 +8,13 @@ mod hands;
 mod triggers;
 mod approvals;
 mod orchestration;
-#[cfg(feature = "multi-agent")]
 mod a2a;
 
 use std::sync::Arc;
 use tokio::sync::{broadcast, Mutex};
 use zclaw_types::{Event, Result, AgentState};
 
-#[cfg(feature = "multi-agent")]
 use zclaw_types::AgentId;
-#[cfg(feature = "multi-agent")]
 use zclaw_protocols::A2aRouter;
 
 use crate::registry::AgentRegistry;
@@ -56,11 +53,9 @@ pub struct Kernel {
     mcp_adapters: Arc<std::sync::RwLock<Vec<zclaw_protocols::McpToolAdapter>>>,
     /// Dynamic industry keyword configs — shared with Tauri frontend, loaded from SaaS
     industry_keywords: Arc<tokio::sync::RwLock<Vec<zclaw_runtime::IndustryKeywordConfig>>>,
-    /// A2A router for inter-agent messaging (gated by multi-agent feature)
-    #[cfg(feature = "multi-agent")]
+    /// A2A router for inter-agent messaging
     a2a_router: Arc<A2aRouter>,
     /// Per-agent A2A inbox receivers (supports re-queuing non-matching messages)
-    #[cfg(feature = "multi-agent")]
     a2a_inboxes: Arc<dashmap::DashMap<AgentId, Arc<Mutex<adapters::AgentInbox>>>>,
 }
 
@@ -135,7 +130,6 @@ impl Kernel {
         }
 
         // Initialize A2A router for multi-agent support
-        #[cfg(feature = "multi-agent")]
         let a2a_router = {
             let kernel_agent_id = AgentId::new();
             Arc::new(A2aRouter::new(kernel_agent_id))
@@ -159,9 +153,7 @@ impl Kernel {
             extraction_driver: None,
             mcp_adapters: Arc::new(std::sync::RwLock::new(Vec::new())),
             industry_keywords: Arc::new(tokio::sync::RwLock::new(Vec::new())),
-            #[cfg(feature = "multi-agent")]
             a2a_router,
-            #[cfg(feature = "multi-agent")]
             a2a_inboxes: Arc::new(dashmap::DashMap::new()),
         })
     }