Pass `.gitmodules` to the manifests parser so submodule URLs appear in `dependencies[]`

[andrew]

knowledge/_shared/gitsubmodules.toml lists .gitmodules under [detect] files and [config] files, so brief reports the Git Submodules tool. .gitmodules is not in knowledge/_shared/_manifests.toml [manifests] files, so detect.go never hands it to manifests.Parse, and the submodule URLs never reach dependencies[].

$ brief --json https://github.com/alexcrichton/openssl-src-rs | jq '[.manifests[].path], [.dependencies[]|select(.purl|startswith("pkg:git"))]'
["Cargo.toml",".github/workflows/main.yml","testcrate/Cargo.toml"]
[]

git-pkgs/manifests/internal/git/git.go already registers gitmodulesParser for .gitmodules, so adding ".gitmodules" to _manifests.toml [manifests] files should surface each submodule as a pkg:git/... dependency with no code change.