Rewrite codex-maintenance.sh to scope CI checks to changed files

CI was running a full eval of every host + package on every push/PR,
which was slow enough to routinely time out the Gitea runner. Default
mode now diffs against a base ref and scopes nixpkgs-fmt/statix/eval to
the files that changed and the hosts/packages they can affect; a change
to flake.nix/flake.lock/variables.nix/modules/common/* (or any other
modules/*.nix outside platforms//build-types, whose blast radius isn't
inferable from the path) falls back to evaluating everything. The old
full sweep moves behind --full-check, which CI never passes; --dry-run
adds build-planning on top of whichever scope is active.

Also trims codex-setup.sh's redundant full host eval loop -- that's
what codex-maintenance.sh is for; setup should just install tooling.

scripts/codex-setup.sh

@@ -82,13 +82,6 @@ if ! command -v jq >/dev/null 2>&1; then
 fi
 
 echo "Available NixOS hosts:"
-hosts="$(list_flake_targets .)"
-echo "$hosts"
+list_flake_targets .
 
-echo "Evaluating all host toplevel derivations..."
-for host in $hosts; do
-  echo "==> Evaluating $host"
-  nix eval --raw "${NIX_EVAL_FLAGS[@]}" ".#nixosConfigurations.${host}.config.system.build.toplevel.drvPath"
-done
-
-echo "Codex setup complete."
+echo "Codex setup complete. Run bash scripts/codex-maintenance.sh to validate changes."