docs: add open-item rate limit and UI/UX media rules to CONTRIBUTING

[anderdc]

Summary

Adds an "Automatic Closures" section to CONTRIBUTING.md documenting two bot-enforced rules:

• Open item limits. Each contributor capped at 2 open PRs and 2 open issues; a 3rd of either type is auto-closed on submission. Limits apply independently.
• UI/UX changes require before/after media. PRs touching rendered UI without before/after screenshots or recordings are liable to be closed without review. Pure logic changes are exempt.

The maintainer bot (agentic-maintainer) needs a follow-up change to drop its threshold from 3 to 2 to match.

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Checklist

• `npm run format` and `npm run lint:fix` have been run
• Docs-only — no build or screenshot impact