feat(mobile): add branch and pull request picker

[jln13x]

Summary

• add a searchable iOS checkout picker modeled on the desktop branch selector
• list local and remote refs, reuse existing checkouts, and prepare isolated worktrees for branches that are not checked out
• show a Checkout PR action when the search contains a pull request number, URL, or checkout command, using the existing resolve/prepare workflow
• keep the mobile behavior behind the existing checkout-aware thread creation flag and document the fork customization

Verification

• vp check
• vp run typecheck
• vp run lint:mobile
• vp test run apps/server/src/sourceControl/GitHubCli.test.ts apps/mobile/src/features/threads/newTaskCheckoutSelection.test.ts (17 tests)
• verified in the iOS Simulator with repository branches, branch search, manual pull request references, and current-checkout selection