`docs/scheduler/ci.md` told the reader to run `nix develop -c treefmt`.
treefmt is not on the devshell PATH — `nix develop -c which treefmt` exits
1 while `which cargo` resolves, so the check discriminates and the
documented command has never worked from that shell.
The same line enumerated the formatter set as "rustfmt + nixfmt + taplo",
which was already wrong before this branch: keep-sorted has been enabled
all along, and the commit before this adds prettier. Replaced the list
with a pointer to nix/treefmt.nix so it can't drift out of sync again.
Also drops "+ rustfmt" from the nix-flake-check row: rustfmt is one of
treefmt's formatters, not a separate thing that runs alongside it.
36b67fcda3