setup-pstack compatibility alias

Portability (required)

1. Read ../setup-ystack/SKILL.md in full.
2. Treat /setup-pstack as /setup-ystack.
3. When a legacy pstack-models.* file exists, use it as migration input and write the preferred ystack-models.* path.
4. Do not delete legacy configuration unless the user explicitly asks.

Report both the new path and any legacy path that was read.