Refactor GitHub client into domain-specific files

[thomas-vilte]

Description

I split the monolithic client.go file in internal/vcs/github into smaller, domain-specific files to improve maintainability and readability. The logic remains unchanged, but the code is now organized by domain:

• client_issues.go: Handles issue-related operations (fetching, creating, and listing issues).
• client_labels.go: Handles repository labels (fetching, creating, and validating labels).
• client_pr.go: Handles pull request operations (updating PRs, fetching PR data, and getting diffs).
• client_release.go: Handles release operations (creating, updating, and uploading binaries).

This restructuring makes the GitHub client easier to navigate and maintain as the project grows.

Fixes #

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

This is a pure refactoring of file structures without functional changes. Existing unit and integration tests should be run to verify that no behavior was broken.

• Unit tests
• Integration tests
• Manual test: (describe below)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Test Plan & Evidence

Suggested Manual Verification

• Unit Tests: Run go test ./... and ensure all tests pass
• No Regressions: Verify that related features still work as expected