docs: drop the stale "just test exit code" caveat

`just test` has enforced the suite result since #350: the recipe raises unless
`evalTests` is `true`, so a zero exit already means pass. Update the places that
still described the old raw-`nix eval` behaviour: AGENTS.md, README.md,
SECURITY.md, outputs/README.md, and the nix-config-debug / nix-config-new-host /
nix-config-update skills.

The exit-0 caveat is kept only where the raw `nix eval .#evalTests` command is
documented, because that command genuinely exits 0 even on `false`.

outputs/README.md

@@ -31,9 +31,13 @@ set for each NixOS and nix-darwin host.
 How to run all the eval tests:
 
 ```bash
-nix eval .#evalTests --show-trace --print-build-logs --verbose
+just test
 ```
 
+`just test` exits non-zero unless the suite returns `true`. The underlying
+`nix eval .#evalTests --show-trace --print-build-logs --verbose` prints the result but exits 0 even
+on `false`, so read its output when you need the failing trace.
+
 Each test is a directory `outputs/<system>/tests/<name>/` holding `expr.nix` and `expected.nix`; the
 suite passes when every `expr` equals its `expected`.