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

tests/fixtures/flake_parts_autochecks/flake.nix

@@ -0,0 +1,33 @@
+{
+  outputs = {
+    flake-parts,
+    systems,
+    ...
+  } @ inputs:
+    flake-parts.lib.mkFlake {inherit inputs;} {
+      imports = [
+        "@repo_path@/lib/flakeModule.nix"
+      ];
+      systems = import systems;
+      flake = {};
+      perSystem = {pkgs, ...}: let
+        fakeCheck = pkgs.runCommand "fake-check" {} "touch $out";
+      in {
+        checks = {
+          my-check = fakeCheck;
+        };
+        ci = {
+          config.autoChecks.enable = true;
+          pipelines.default = {
+            stages = ["test"];
+          };
+        };
+      };
+    };
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+    systems.url = "github:nix-systems/default-linux";
+  };
+}