fix(core): take over agent, reuse callbacks

- Kill running agents before replace
- Retry copy with backoff while locked
- Reuse syscall callbacks at package scope
- Add callback regression test

lib/instance/kill_stub.go

@@ -0,0 +1,5 @@
+//go:build !windows
+
+package instance
+
+func KillOtherAgents() error { return nil }