Document name-redefinition and setup-cell constraints

[manzt]

Closes #47

[Copilot]

Pull request overview

This PR updates the marimo-pair skill documentation to reduce common notebook authoring errors, specifically around public name redefinition across cells and constraints of the setup cell.

Changes:

• Added explicit guidance that each public name should be defined in exactly one owning cell (with a pointer to gotchas).
• Documented setup cell constraints (must be self-contained; cannot reference names defined in other cells) and clarified how to modify an existing setup cell.
• Expanded gotchas.md with a dedicated section on cross-cell name redefinition and updated the references list description.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
skills/marimo-pair/SKILL.md	Adds an explicit “define each public name once” rule and updates the gotchas reference blurb.
skills/marimo-pair/reference/notebook-improvements.md	Adds setup-cell constraints and clarifies how to handle an already-existing setup cell.
skills/marimo-pair/reference/gotchas.md	Adds a new gotcha section explaining public name redefinition failures and how to resolve them.

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