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>

desktop/src-tauri/src/lib.rs

@@ -146,6 +146,9 @@ pub fn run() {
             kernel_commands::skill::skill_create,
             kernel_commands::skill::skill_update,
             kernel_commands::skill::skill_delete,
+            // Orchestration commands (multi-skill graphs)
+            kernel_commands::orchestration::orchestration_execute,
+            kernel_commands::orchestration::orchestration_validate,
             // Hands commands (autonomous capabilities)
             kernel_commands::hand::hand_list,
             kernel_commands::hand::hand_execute,