refactor(advance_phase): simplify previous_phase capture

Remove over-engineered _previous_value static method that did index
math on a hardcoded phase list. Instead, capture the previous phase
before the transition — clearer intent, fewer lines, same behavior.

ce-simplify-code step of LFG pipeline.

src/agentkit/tools/advance_phase.py

@@ -56,6 +56,8 @@ class AdvancePhaseTool(Tool):
         self._engine = engine
 
     async def execute(self, **kwargs) -> dict[str, Any]:
+        # Capture previous phase before transition (engine is single-threaded per request).
+        previous = self._engine.current_phase
         new_phase = self._engine.advance_phase()
         if new_phase is None:
             # Either no policy set, or already at DELIVERY.
@@ -74,26 +76,7 @@ class AdvancePhaseTool(Tool):
             }
         return {
             "is_error": False,
-            "previous_phase": self._previous_value(new_phase),
+            "previous_phase": previous.value if previous else "",
             "current_phase": new_phase.value,
             "message": f"Phase advanced to {new_phase.value}.",
         }
-
-    @staticmethod
-    def _previous_value(current: Any) -> str:
-        """Return the previous phase value (for telemetry/UI)."""
-        from agentkit.core.phase import PhaseState
-
-        order = [
-            PhaseState.PLANNING,
-            PhaseState.BUILDING,
-            PhaseState.VERIFICATION,
-            PhaseState.DELIVERY,
-        ]
-        try:
-            idx = order.index(current)
-        except ValueError:
-            return ""
-        if idx == 0:
-            return ""
-        return order[idx - 1].value