refactor(desktop): simplify state management with derived state pattern

[iamdin]

Summary

• Rename selected* to current* (currentRepositoryId, currentTaskId)
• Remove selectedWorktreeId (derive from task instead)
• Rename openedWorktreeIds to worktreeTerminalIds
• Use useMemo to derive currentRepository instead of syncing with useEffect
• Remove unused use-selected-worktree hook
• Add state management best practices to CLAUDE.md

Key Principle

Don't Sync State. Derive It! - Never use useEffect to synchronize state between different sources. Instead, derive values at render time using useMemo.

Test plan

• Verify repository selection works correctly
• Verify task selection works correctly
• Verify terminal sessions persist when switching tasks
• Verify fallback to first repository when stored ID is invalid