git wip save should handle submodules

[bartman]

currently submodules are not handled correctly...

❯ git wip save -i
[2026-05-14 09:08:56.138] [error] cannot stage changes: invalid path: 'build/_deps/spdlog-src/'

should probably have some options to steer it in the right direction:

git wip save -s --submodules           # full submodule WIP
git wip save -S --no-submodules        # skip submodules
git wip save --ref-submodules          # store submodule ref in this repo only
git wip save --in-submodules           # recurse into submodules