Add a Git submodule and track a pinned version of another repository

Documented steps

1. Run `git submodule add <repository-url> <path>` from your superproject root; this clones the repo into <path>, stages it in the index, and records the URL in a `.gitmodules` file.
2. For a relative URL to a sibling repo, use `../foo.git` (not `./foo.git`) — Git resolves submodule URLs like relative directories.
3. Commit the superproject change: `git commit -am "add submodule"` — this records the exact commit SHA the submodule is pinned to, plus the `.gitmodules` entry.
4. On a fresh clone, run `git submodule update --init` (or `git clone --recurse-submodules <url>`) to fetch and check out the pinned submodule commits; `--recursive` handles nested submodules.
5. Set the default remote-tracking branch a submodule follows with `git submodule set-branch -b <branch> <name>`, and change its URL later with `git submodule set-url <name> <newurl>` (which also syncs the remote config).
6. Check per-submodule status with `git submodule status` (a leading `-` means not initialized, `+` means the checked-out commit differs from the index SHA).
7. To detach a submodule permanently, use `git rm <path>` (after `git submodule deinit -f <path>` for older workflows) rather than only editing `.gitmodules`.
8. Official docs: https://git-scm.com/docs/git-submodule