Context debug: guardar system_prompt + messages completos del último build

El endpoint /context-debug ahora devuelve full_context con el
system_prompt y messages exactos enviados al modelo.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/context/engine.py

@@ -52,6 +52,8 @@ class ContextEngine:
         # Debug history: last N context builds per session
         self._history: dict[str, list[dict[str, Any]]] = defaultdict(list)
         self._max_history = 20
+        # Full context of the LAST build per session (not accumulated)
+        self._last_full_context: dict[str, dict[str, Any]] = {}
 
     # ------------------------------------------------------------------
     # Public — build context for a model call
@@ -117,6 +119,14 @@ class ContextEngine:
             total_token_estimate=total_tokens,
         )
 
+        # Guardar contexto completo del último build (solo el último por sesión)
+        self._last_full_context[session.session_id] = {
+            "system_prompt": system_prompt,
+            "messages": messages,
+            "total_tokens": total_tokens,
+            "timestamp": time.time(),
+        }
+
         # --- Debug: log and store context build ---
         section_summary = []
         for s in sections:
@@ -169,6 +179,10 @@ class ContextEngine:
         history = self._history.get(session_id, [])
         return history[-1] if history else None
 
+    def get_last_full_context(self, session_id: str) -> dict[str, Any] | None:
+        """Return the full context (system_prompt + messages) of the last build."""
+        return self._last_full_context.get(session_id)
+
     def rehydrate_artifact(
         self,
         artifact: ArtifactSummary,