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/bump-nixpkgs-release.sh

@@ -129,5 +129,6 @@ echo "flake.lock still points at the old input revisions until refreshed. Either
 echo "  nix flake update nixpkgs home-manager   # just these two inputs"
 echo "  nix flake update                        # everything — see docs/flake-lock-automation.md"
 echo
-echo "Then run 'bash scripts/codex-maintenance.sh dry-run' before committing —"
-echo "a channel bump can shift option defaults across every host."
+echo "Then run 'bash scripts/codex-maintenance.sh --full-check --dry-run' before"
+echo "committing — a channel bump can shift option defaults across every host,"
+echo "and only --dry-run actually builds anything to catch that."