pstack Codex port rules

Preserve the original MIT license and attribution.
Treat https://github.com/cursor/plugins/tree/main/pstack as upstream.
Keep frontmatter to name and description.
Do not introduce Cursor-only paths, tools, model slugs, or agent types.
Codex collaboration agents inherit the session runtime unless the active tool schema says otherwise.
Keep external writes inside the user's explicit scope.
Run ./scripts/audit.py before publishing changes.
Update UPSTREAM_COMMIT only after the corresponding upstream changes have been reviewed and ported.

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