Add stacker_root path + config key
Phase 1 of the stacker fork needs a dedicated directory for its sqlite db
and ops-ownership sentinel. Introduce stacker_root on Paths (default
~/.stacker, overridable via [paths].stacker_root in pm config), with
derived helpers stacker_db() and stacker_ops_marker(). Extend pm_env so
every test gets an isolated stacker_root alongside repos/worktrees/projects.