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
