fix: move PlanningMode import inside test module

PlanningMode was imported at lib level but only used in tests,
causing unused import warning in CI with -D warnings.

Author: claude

core/src/store.rs

@@ -381,6 +381,7 @@ mod tests {
     use super::*;
     use crate::hitl::ConfirmationPolicy;
     use crate::permissions::PermissionPolicy;
+    use crate::prompts::PlanningMode;
     use crate::queue::SessionQueueConfig;
     use tempfile::tempdir;