feat(environment): add step() skeleton with active episode guard

server/environment.py

@@ -54,3 +54,12 @@ class RedTeamEnvironment:
             feedback                = "Episode started. Begin your attack strategies.",
             episode_id              = self.episode_id,
         )
+
+    async def step(self, action: AttackAction) -> StepResult:
+        if not self.is_active:
+            raise ValueError("Episode not active. Call /reset first.")
+        if self.turn >= self.max_turns:
+            raise ValueError("Episode already complete. Call /reset to start a new one.")
+
+        self.turn += 1
+        return None # Placeholder