Memoize detect_github_protocol to avoid repeated subprocess calls

[raiderrobert]

Context

In src/template/source.rs (lines 25-40), detect_github_protocol shells out to gh on every gh: abbreviation expansion:

fn detect_github_protocol() -> String {
    Command::new("gh")
        .args(["config", "get", "git_protocol", "-h", "github.com"])
        .output()
        ...
}

This also makes test behavior non-deterministic — tests must accept either SSH or HTTPS results.

Suggestion

Memoize with OnceLock or LazyLock so the subprocess is only called once per process.