Exit plan mode when plan is completed or blocked

When a plan reaches a terminal state (all items done or blocked) in
interactive mode, automatically exit plan mode and return to normal
prompt.

Changes:
- Add Agent::is_plan_terminal() method to check if plan is complete
- Add check_and_exit_plan_mode_if_terminal() helper in interactive.rs
- Call the helper after each execute_user_input() to detect completion

Fixes issue where plan mode prompt ' >> ' persisted after plan completion.

crates/g3-core/src/lib.rs

@@ -50,7 +50,7 @@ pub use skills::{Skill, discover_skills, generate_skills_prompt};
 #[cfg(test)]
 mod task_result_comprehensive_tests;
 use crate::ui_writer::UiWriter;
-use tools::plan::{check_plan_approval_gate, ApprovalGateResult};
+use tools::plan::{check_plan_approval_gate, read_plan, ApprovalGateResult};
 
 #[cfg(test)]
 mod tilde_expansion_tests;
@@ -1548,6 +1548,22 @@ impl<W: UiWriter> Agent<W> {
         self.in_plan_mode
     }
 
+    /// Check if the current plan is in a terminal state (all items done or blocked).
+    ///
+    /// Returns true if:
+    /// - A plan exists AND all items are in terminal state (done or blocked)
+    ///
+    /// Returns false if:
+    /// - No session_id is set
+    /// - No plan exists for the session
+    /// - Plan has items that are not terminal (todo or doing)
+    pub fn is_plan_terminal(&self) -> bool {
+        let Some(session_id) = &self.session_id else {
+            return false;
+        };
+        read_plan(session_id).ok().flatten().map_or(false, |plan| plan.is_complete())
+    }
+
     // =========================================================================
     // STREAMING & LLM INTERACTION
     // =========================================================================