Isabelle2024 has added a feature to set output to cartouches, at least for sledgehammer and sketch-and-explore outlines:

Using declare [[atp_proof_cartouches]], the output will render with ‹...› instead of "...":

[...]
  imports
    "HOL-ex.Sketch_and_Explore"
[...]

declare [[atp_proof_cartouches]]

lemma ‹reflp (=)›
sketch

--- output ---

proof
  fix x :: 'b
  show ‹x = x›
   sorry
qed

Also note, how the outlines are more natural (fix instead of for) thanks to the improvements to sketch-and-explore.