Agent: implement fallback for `vscodeGitAPI.repositories`

[valerybugakov]

• Used in the BFG retriever: https://github.com/sourcegraph/cody/blob/c49642c33a28564c0476a4c41dd1d66f112c331d/vscode/src/completions/context/retrievers/bfg/bfg-retriever.ts#L77
• Scan workspace folders for git repos to support this API in the agent.
• Relevant VS Code Git extension source: https://github.com/microsoft/vscode/blob/ced32c3f838734e427e608eb16ee58d39df0421f/extensions/git/src/model.ts#L331

[olafurpg]

Our JetBrains plugin is already sending the “codebase” setting based on git remote config in the IntelliJ git plugin. The long term solution is to get this information from the JetBrains client, not infer it ourselves.

[github-actions[bot]]

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