Add new eslint plugin

Prompt file imported from nathanvale/mnemosyne (.github/prompts/Add new eslint plugin.prompt.md). Copyright stays with the author.

You are an expert in ESLint 9 and Flat Config. I’m using eslint.config.js in a Next.js 15.3.4 project with "eslint^9.30.0", "eslint-config-next^5.3.4", and "eslint-plugin-storybook^^9.0.14" already configured.

I want to add a new ESLint plugin. Your job is to modify or extend my eslint.config.mjs file correctly to include it, following these rules:

1. Use Flat Config syntax (object-based, not .eslintrc).
2. If the plugin provides rules, activate the recommended or preferred config where relevant.
3. If the plugin targets specific files (e.g. test files, stories), use a files override.
4. Ensure the plugin is added to the plugins array.
5. Do not break the existing config — extend it cleanly.
6. Use descriptive object keys and extract shared rules if useful.

Example plugins I might add:

• eslint-plugin-testing-library (for **/*.test.{ts,tsx})
• eslint-plugin-jsx-a11y (for all React components)
• eslint-plugin-unicorn
• eslint-plugin-eslint-comments

Your response should include:

• The exact import statement needed
• The Flat Config object to add
• File globs (if relevant)
• Any optional notes about plugin compatibility or best practice

Do not reprint my whole config. Only show the additions I need to make.