host: add supervisor-gated clean core.restart RPC

Return success before scheduling the existing shutdown path, coalesce pending
requests, and mark #loop invocations with SAUSAGE_LOOP=1. Keep core.exit
compatibility and document explicit per-call approval for all agents.

Card: SAU-134

cli/tests/plugins/probe/src/main.rs

@@ -447,6 +447,21 @@ fn dispatch(request: &Value, lines: &mut Lines) -> Option<Value> {
             burst(request);
             return None;
         }
+        "restart" => {
+            assert_eq!(
+                lock(&json!({"path":"/state/restart-stop"}), lines),
+                json!({"held":true})
+            );
+            assert_eq!(
+                outgoing_call(&json!({"method":"core.restart", "params":{}}), lines),
+                json!({})
+            );
+            std::fs::write("/state/restart-answer", "received").unwrap();
+            serve(request, lines);
+            std::fs::write("/state/restart-stop", "persisted after EOF").unwrap();
+            eprintln!("restart stop hook finished");
+            return None;
+        }
         "serve" => {
             serve(request, lines);
             return None;