feat(kernel): add multi-skill orchestration bridge + true parallel execution

- Kernel orchestration bridge: execute_orchestration, auto_compose_skills,
  validate_orchestration methods on Kernel struct
- True parallel execution: replace sequential for-loop with tokio::JoinSet
  for concurrent node execution within parallel groups
- Tauri commands: orchestration_execute (auto-compose or pre-defined graph),
  orchestration_validate (dry-run validation)
- Full type conversions: OrchestrationRequest/Response with camelCase serde

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

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

@@ -7,6 +7,7 @@ mod skills;
 mod hands;
 mod triggers;
 mod approvals;
+mod orchestration;
 #[cfg(feature = "multi-agent")]
 mod a2a;