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/Cargo.toml

@@ -8,7 +8,7 @@ rust-version.workspace = true
 description = "ZCLAW kernel - central coordinator for all subsystems"
 
 [features]
-default = []
+default = ["multi-agent"]
 # Enable multi-agent orchestration (Director, A2A protocol)
 multi-agent = ["zclaw-protocols/a2a"]