pstack compatibility alias

Portability (required)

1. Read ../ystack/SKILL.md in full before taking any action.
2. Treat /pstack as /ystack and follow the ystack router exactly.
3. Keep this directory's playbooks/ and references/ as shared legacy runtime assets until a separately versioned migration removes them.
4. Prefer /setup-ystack and ystack-named model overrides; continue honoring legacy configuration through the adapters.

Do not present pstack as the public name of this repository. It remains here for upstream attribution and backwards compatibility.