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/adapters.rs

@@ -106,13 +106,11 @@ impl SkillExecutor for KernelSkillExecutor {
 
 /// Inbox wrapper for A2A message receivers that supports re-queuing
 /// non-matching messages instead of dropping them.
-#[cfg(feature = "multi-agent")]
 pub(crate) struct AgentInbox {
     pub(crate) rx: tokio::sync::mpsc::Receiver<zclaw_protocols::A2aEnvelope>,
     pub(crate) pending: std::collections::VecDeque<zclaw_protocols::A2aEnvelope>,
 }
 
-#[cfg(feature = "multi-agent")]
 impl AgentInbox {
     pub(crate) fn new(rx: tokio::sync::mpsc::Receiver<zclaw_protocols::A2aEnvelope>) -> Self {
         Self { rx, pending: std::collections::VecDeque::new() }