- Add listPullRequests and getPullRequest methods to GitHubClient
- Add /github/pulls/fetch and /github/pulls/import API endpoints
- Add apiFetchGitHubPulls and apiImportGitHubPull frontend API functions
- Add tabbed UI for Issues and Pull Requests in GitHubImportModal
- Add comprehensive PR tab tests (14 new tests)
- Include changeset for the new PR tab feature