PR draft icon implemented instead of normal PR icon

[Styrse]

Description

Fix the sidebar icon for draft GitHub pull requests.

Pane now retrieves GitHub's isDraft value and propagates it through the git status cache and
frontend types. Draft PRs use the GitPullRequestDraft icon, while regular PRs continue using the
standard GitPullRequest icon.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the CONTRIBUTING.md guidelines
• My code follows the code style of this project
• I have performed a self-review of my own code
• 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
• I have run pnpm typecheck and pnpm lint locally

Additional Notes

• Draft PR metadata is fetched using the GitHub CLI's isDraft field.
• Regular PR and branch icons are unchanged.
• The focused GitStatusManager suite passes all 23 tests.
• Workspace type-checking and linting pass. Lint reports only existing unrelated warnings.