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/lib.rs

@@ -10,7 +10,6 @@ pub mod trigger_manager;
 pub mod config;
 pub mod scheduler;
 pub mod skill_router;
-#[cfg(feature = "multi-agent")]
 pub mod director;
 pub mod generation;
 pub mod export;
@@ -21,13 +20,11 @@ pub use capabilities::*;
 pub use events::*;
 pub use config::*;
 pub use trigger_manager::{TriggerManager, TriggerEntry, TriggerUpdateRequest, TriggerManagerConfig};
-#[cfg(feature = "multi-agent")]
 pub use director::{
     Director, DirectorConfig, DirectorBuilder, DirectorAgent,
     ConversationState, ScheduleStrategy,
     // Note: AgentRole is intentionally NOT re-exported here — use generation::AgentRole instead
 };
-#[cfg(feature = "multi-agent")]
 pub use zclaw_protocols::{
     A2aRouter, A2aAgentProfile, A2aCapability, A2aEnvelope, A2aMessageType, A2aRecipient,
  A2aReceiver,