require a2a pack write prefix support

agent_builder/builder.py

@@ -218,7 +218,8 @@ Your tools:
                                     for ``@skill`` semantics,
                                     ``context.py`` for ``RunContext``
                                     (workspace, sandbox, emit_progress,
-                                    ask, collect, request_scope),
+                                    ask, collect, request_scope,
+                                    ensure_read/ensure_write),
                                     ``runtime.py`` for ``AgentRuntime``
                                     fields (apt_packages, pricing,
                                     egress, etc.).
@@ -350,6 +351,7 @@ def _build_project_backend(ctx: BuilderContext, *, settings: Settings) -> Any |
             allow_patterns=(f"{prefix}/**", "outputs/**"),
             deny_patterns=(),
             outputs_prefix=None,
+            write_prefixes=(f"{prefix}/", "outputs/"),
         )
     )
     return _with_builder_skills(WorkspaceBackend(workspace))