shellHook now derives PGDATA from $(git rev-parse --show-toplevel)
instead of $PWD. nix develop / direnv from a subdir (e.g.
backend/) used to seed a duplicate cluster at backend/.postgres
that leaked into git tracking. .gitignore loses its leading slash
on .postgres/ + .direnv/ + .pc.* so any nested cluster also gets
ignored. fallback to pwd preserves behavior outside a git repo.
968 stray backend/.postgres/* blobs already pruned from history
via git-filter-repo before this commit.