feat: auto-generate CI jobs from flake checks

Add `checks` option and `config.autoChecks` to auto-generate a CI job
for each flake check derivation. Auto-generated jobs go through
`mkJobPatched` for proper Nix setup, cache, and environment handling.

The `checks` option is automatically wired to `config.checks` in
flake-parts. System is inferred from `pkgs.system` and set via the new
`system` option. User-defined jobs with the same name always take
precedence.

New tests cover both flake-parts and direct API entry points.

MT-14138

lib/default.nix

@@ -20,7 +20,7 @@ args: let
         modules = [
           cilib.modules.nixCiSubmodule
           {
-            inherit config;
+            config = config // {inherit (pkgs) system;};
           }
         ];
       }).config;