Summary

• detect main vs linked worktrees from resolved gitdir metadata instead of checking whether the .git path is a directory
• keep main checkouts created with the separate-git-dir layout protected from clean and nuke flows
• add regression coverage for main worktrees whose .git is a file

Validation

• gofmt -w internal/worktree/git.go internal/worktree/git_test.go
• go test ./internal/worktree/...
• go test ./...