feat: Add repository ruleset code quality support

[tvost2]

Summary

• model the GitHub code_quality repository ruleset rule
• preserve its severity when marshaling and unmarshaling rulesets
• add regression coverage for round-tripping the rule

Fixes #4520

Tests

• go test ./github -count=1

[gmlewis]

@tvost2 - please run step 4 of CONTRIBUTING.md and push the changes to this PR.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.53%. Comparing base (3a439fa) to head (2c92045).

Files with missing lines	Patch %	Lines
github/rules.go	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4523      +/-   ##
==========================================
- Coverage   98.53%   98.53%   -0.01%     
==========================================
  Files         195      195              
  Lines       17848    17862      +14     
==========================================
+ Hits        17587    17600      +13     
- Misses        261      262       +1     

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[tvost2]

Addressed the Codecov coverage comment by adding marshal/unmarshal regression coverage for BranchRules.CodeQuality.

Validation:

• go test ./... -count=1 passes.
• gofmt applied.
• git diff --check passes.
• script/fmt.sh and script/lint.sh could not complete in this Windows environment: the shell wrapper stalled after installing golangci-lint, and the direct linter invocation reports the repository plugin structfield is unavailable locally.

[gmlewis]

• script/fmt.sh and script/lint.sh could not complete in this Windows environment: the shell wrapper stalled after installing golangci-lint, and the direct linter invocation reports the repository plugin structfield is unavailable locally.

Hmm... was this in a WSL or git bash window? If not, could you please try running them in one of these windows?
I don't have a Windows box available, and we have not had this report before, so I'm just guessing here.

[tvost2]

Updated the PR after the maintainer feedback:

• ran generation in Git Bash and committed the generated CodeQuality accessors/tests;
• added invalid-parameter regressions for repository and branch rules to cover the remaining Codecov misses;
• focused ruleset tests pass and git diff --check passes.

The official Git Bash wrapper reaches the repository scripts, but this Windows checkout cannot execute the downloaded custom-gcl path. script/generate.sh --check now gets past the generated Go files and reports only the local toolchain changing go.sum during tidy; no dependency files were included in this PR.

[gmlewis]

@tvost2 - note that issue #4520 is assigned to someone else anyway so we will wait for their response.
As covered in CONTRIBUTING.md, please sign up to address open issues before posting a PR to prevent application of effort.