Add Close PR command palette action

[khoi]

• add a new command palette action Close PR for open pull requests and wire it through item IDs, delegates, and palette row rendering/help text
• route Close PR from AppFeature into repositories pull request actions
• add PullRequestAction.close in RepositoriesFeature with GitHub availability checks, close/in-progress+success toasts, and immediate + delayed PR refresh
• extend GithubCLIClient with closePullRequest backed by gh pr close <number>
• add regression coverage for command-palette visibility rules (Close PR shown for open, hidden for merged), app delegate dispatch, and repositories close-action behavior
• validate with make check, targeted xcodebuild test -only-testing:supacodeTests/CommandPaletteFeatureTests -only-testing:supacodeTests/AppFeatureCommandPaletteTests -only-testing:supacodeTests/RepositoriesFeatureTests, and make build-app