Repository ruleset rule type `code_quality` is not modelled

### Summary

The repository ruleset rule type `code_quality` is not modelled by `RepositoryRule` / the ruleset rule types, so it cannot be read or written through this library. It corresponds to the **"Require code quality results"** checkbox on a branch ruleset, which reached general availability on 2026-07-20.

### API shape

`GET /repos/{owner}/{repo}/rulesets/{id}` returns it as a rule alongside the existing types:

```json
{
"id": 16352232,
"name": "Project branch",
"target": "branch",
"enforcement": "active",
"rules": [
{ "type": "code_quality", "parameters": { "severity": "errors" } },
{ "type": "copilot_code_review", "parameters": { "review_on_push": true, "review_draft_pull_requests": false } }
]
}
```

`severity` is the lowest severity level at which code quality results block a merge. I have only observed `"errors"` on the wire, so the full accepted set is worth confirming against the docs before fixing an enum — note the existing `code_scanning` rule's `security_alerts_threshold` already uses `"errors"` / `"errors_and_warnings"` style values in `rules.go`, which may or may not be the same vocabulary here.

### Why it matters beyond a missing field

`PUT /repos/{owner}/{repo}/rulesets/{id}` replaces the entire `rules` array. Because the rule cannot be represented, any consumer that round-trips a ruleset — read, modify one unrelated attribute, write back — will silently drop `code_quality` and remove a merge gate, with nothing in the request or response to indicate it happened.

That is not hypothetical. `integrations/terraform-provider-github` builds its `github_repository_ruleset` resource on this library, and a ruleset carrying this rule imports cleanly, plans cleanly, and would lose the rule on the first apply that touched any other attribute of the resource. We ended up dropping the affected ruleset from Terraform management entirely rather than hold that risk. Modelling the type here unblocks the provider side (there is no provider-side workaround, since there is nothing to serialise).

### Environment

- go-github: absent as of current `master` — `github/rules.go` models `copilot_code_review`, `required_code_scanning`, `required_status_checks` et al., and contains no occurrence of `code_quality` or `CodeQuality`
- Observed against: GitHub.com, private repository in an organisation on the Team plan

### Willing to contribute

Happy to open a PR adding the rule type if a maintainer confirms the preferred shape — in particular whether `severity` should be a plain `string` or a typed constant set.