9p: delegate session tmpdir cleanup to core.Close()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/p9/server.go

@@ -1384,7 +1384,7 @@ func (s *Server) killSession(id string) {
 	s.mu.Unlock()
 	if sess != nil {
 		sess.cancel()
-		os.RemoveAll("/tmp/ollie/" + id) //nolint:errcheck
+		sess.core.Close()
 		fmt.Fprintf(os.Stderr, "olliesrv: killed session %s\n", id)
 	}
 }