fix: remove unused PlanningMode lib-level import

The PlanningMode import was only used in tests, not in lib code.
Having it at lib level caused unused import warning in CI.

Author: claude

core/src/store.rs

@@ -31,7 +31,6 @@
 
 use crate::llm::{Message, TokenUsage, ToolDefinition};
 use crate::planning::Task;
-use crate::prompts::PlanningMode;
 use crate::session::{ContextUsage, SessionConfig, SessionState};
 use anyhow::{Context, Result};
 use serde::{Deserialize, Serialize};