Get a repo's contributor stats

Read per-contributor commit totals for a repository, and handle GitHub’s asynchronous-computation case correctly.

Steps

Call get_repo_contributor_stats with the repository’s owner and name:
```
get_repo_contributor_stats { owner: "octocat", repo: "example" }
```
Check the computing field before reading contributors:
```
{
  "computing": false,
  "contributors": [
    { "login": "octocat", "total": 214 },
    { "login": null, "total": 3 }
  ]
}
```
A null login means GitHub couldn’t map that commit author to a GitHub account. Each entry’s total is that contributor’s all-time commit count on this repository.
If computing is true, GitHub is still computing the stats after a cache miss and contributors will be an empty array — this is not the real answer. Wait a few seconds and call the same tool again with the same owner/repo until computing comes back false.

missing_scope: no GitHub token was resolvable. Set GITHUB_TOKEN or run gh auth login.
github_api_error with a 403: a genuine permission denial (rate-limited 403s are retried automatically up to three times first).