Report Git submodules as dependencies

[andrew]

Adds .gitmodules to manifest discovery so the existing Git parser reports submodule URLs in manifests[] and dependencies[] as direct pkg:git dependencies.

Closes #151

[Copilot]

Pull request overview

This PR enables Git submodules to be surfaced as dependency manifests by adding .gitmodules to the knowledge-base manifest discovery list, so the existing manifests parser can emit submodule URLs into manifests[] and dependencies[] as direct pkg:git dependencies.

Changes:

• Add .gitmodules to knowledge/_shared/_manifests.toml so detect.Engine.manifestPaths() includes it in dependency parsing.
• Add a detection regression test ensuring .gitmodules is reported as a git manifest and its submodules appear as direct runtime Git dependencies.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
knowledge/_shared/_manifests.toml	Adds .gitmodules to the global manifest discovery list so it gets passed into dependency parsing.
detect/detect_test.go	Adds a regression test verifying .gitmodules is included in manifests[] and yields direct pkg:git dependencies for submodules.

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