Document one CellFence error using an existing reproduction — no assignment needed #46

Pick one rule and write one short walkthrough

You do not need to discover a bug or invent a new reproduction. Start from an existing fixture and add its explanation to docs/rules.md. A short section for one rule is enough.

Deliverable

Add one explanation containing:

• A link to the reproduction and the relevant source/manifest paths.
• The contract being violated and why the shown code triggers the rule.
• The command used to reproduce it and the observed rule ID.

Link the walkthrough from the rule's existing table row. An improvement to an already-written walkthrough is equally welcome. Manual correction examples are optional and must be verified; do not change policy just to hide a violation.

From the repository root, install dependencies and build with npm ci and npm run build. For the first row, the CLI invocation is:

node packages/cli/dist/index.js check --root fixtures/invalid/private-cross-cell-import --format markdown

The fixture is intentionally invalid: report its actual failing check and rule ID. For another row, substitute its fixture path.

Done when: the walkthrough agrees with the current contract, its reproduction was checked, links work, and git diff --check passes. Follow CONTRIBUTING.md · AGENTS.md and report actual validation.

Multiple contributors welcome, including AI agents and bots. No assignment, reservation, or advance comment is needed. Check existing files and open PRs, fork the repository, and submit one useful improvement per PR. Different improvements to the same subject remain welcome. No deadline or contributor limit. Use Refs #46; this issue stays open after individual PRs merge.

This is documentation work; changing engine diagnostics belongs in a separate PR. Fixture-specific README work can use #45; do not submit duplicate PRs.