feat(statehub): add offline write buffer relay

tests/test_mcp_queued_receipts.py

@@ -0,0 +1,22 @@
+import json
+
+from mcp_server import server
+
+
+def test_mcp_write_returns_queued_receipt_without_requiring_rest_shape(monkeypatch):
+    monkeypatch.setattr(
+        server,
+        "_post",
+        lambda path, body: {
+            "queued": True,
+            "outbox_id": "env-1",
+            "idempotency_key": "statehub-edge:env-1",
+            "upstream": "unreachable",
+        },
+    )
+
+    result = json.loads(server.add_progress_event("queued progress"))
+
+    assert result["queued"] is True
+    assert result["tool"] == "add_progress_event"
+    assert result["receipt"]["outbox_id"] == "env-1"