feat(statehub): add offline write buffer relay

mcp_server/TOOLS.md

@@ -82,6 +82,13 @@ succeeds but its automatic progress event fails, the tool returns an error with
 the successful `write_result` included so the caller can avoid duplicating the
 entity while recording the missing progress event.
 
+When API_BASE points at the optional State Hub edge relay and the central API is
+unreachable, queueable write tools may return a queued receipt instead of the
+normal REST shape. The receipt means the local outbox accepted the write; it is
+not yet a central commit. Automatic progress-event side effects are skipped for
+queued primary writes so replay does not duplicate records. Operators can inspect
+and replay with statehub outbox status and statehub outbox replay.
+
 ---
 
 ## Query Tools (read-only, use freely)