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.

docs/flake-lock-automation.md

@@ -8,9 +8,14 @@ and to verify that declared NixOS hosts still evaluate after dependency updates.
 - A scheduled workflow runs `nix flake update` once per week.
 - On GitHub, any resulting `flake.lock` change is proposed through a pull request.
 - On Gitea, the workflow can commit and push `flake.lock` directly when PR automation is not configured.
-- A separate CI workflow evaluates every configured host before merge, listed
-  dynamically via `nix eval --json .#nixosConfigurations --apply builtins.attrNames`
-  rather than hand-enumerated, so it can't drift as `<platform>-<buildtype>`
+- A separate CI workflow runs `scripts/codex-maintenance.sh` before merge.
+  Its default mode scopes eval to the hosts/packages a change can affect,
+  determined from a git diff against the PR base — but a `flake.lock` change
+  is treated as repo-wide and always falls back to evaluating every host, so
+  a lock-file update PR still gets full coverage. Hosts are still listed
+  dynamically via
+  `nix eval --json .#nixosConfigurations --apply builtins.attrNames` rather
+  than hand-enumerated, so that fallback can't drift as `<platform>-<buildtype>`
   targets are added or removed. See `README.md` for the current target list.
 
 ## Why hosts should stop using `--upgrade-all`