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/director.rs

@@ -448,6 +448,9 @@ Respond with ONLY the number (1-{}) of the agent who should speak next. No expla
             temperature: Some(0.3),
             stop: vec![],
             stream: false,
+            thinking_enabled: false,
+            reasoning_effort: None,
+            plan_mode: false,
         };
 
         match driver.complete(request).await {