Adds WorktreeModalView/ViewModel showing git status --porcelain as a
recursive file tree with M/A/D/? status badges. Wires the Worktree
button in AgentStripView to OpenWorktreeCommand on DetailsIslandViewModel.
Adds GetStatusPorcelainAsync to GitService.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>