fix(kernel): enable multi-agent compilation + A2A routing tests

- director.rs: add missing CompletionRequest fields (thinking_enabled,
  reasoning_effort, plan_mode) for multi-agent feature gate
- agents.rs: remove unused AgentState import behind multi-agent feature
- lib.rs: replace ambiguous glob re-export with explicit director types,
  resolving AgentRole conflict between director and generation modules
- a2a.rs: add 5 integration tests covering direct message delivery,
  broadcast routing, group messaging, agent unregistration, and
  expired message rejection (10 total A2A tests, all passing)
- Verified: 537 workspace tests pass with multi-agent feature enabled

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -1,6 +1,6 @@
 //! Agent CRUD operations
 
-use zclaw_types::{AgentConfig, AgentId, AgentInfo, AgentState, Event, Result};
+use zclaw_types::{AgentConfig, AgentId, AgentInfo, Event, Result};
 
 #[cfg(feature = "multi-agent")]
 use std::sync::Arc;