[Feature request] Check if the patches can be applied by git am

Hello guys,

I propose to have option in CI/CD to check if the patches can be applied by git am, which will follow what is exactly written here: https://github.com/openwrt/openwrt/blob/df465b6513053c5f14cf26063979f4024ff56ac9/target/linux/generic/PATCHES#L16

I've noticed not only in packages feed (thats the primary use case for me), but also in core repository that there are some patches, wihch does not have proper subject, commit description and Signed-off-by line.

In the past, @feckert in https://github.com/openwrt/openwrt/pull/10264 tried to add missing patch headers, which leads to following issue https://github.com/openwrt/openwrt/issues/10272.

I think in the beginning it is going to be burden, however, with some first potentional issues, it will stay like it is in the current situation as it still happens e.g. https://github.com/openwrt/openwrt/commit/44ef34350095c86227ea0cf84b816258c5195e29 , https://github.com/openwrt/openwrt/commit/3c3614cec4f4355a7c2f7fa5bc61f8bb07e55025 , but overally it will improve the current state.

Start with the referenced OpenWrt target/linux/generic/PATCHES guidance and the repository’s CI/CD entry points. Determine how a workflow can run git am against package-feed and core patches while checking subject, description, and Signed-off-by headers. Done means the CI check reliably reports invalid patches without breaking valid builds.