feat: add content-guidelines skill (MetaMask mobile)

[alidotforrest]

Description

Adds a content-guidelines skill under domains/ui/skills/ that encodes the MetaMask Content Design Style Guide for AI-assisted development. Covers sentence case, punctuation, voice and tone, per-component copy patterns, words to avoid, and the approved terminology list.

Moved from MetaMask/metamask-mobile#35256 at the request of @georgewrmarshall. That PR is now closed in favour of this one.

Related: content guidelines Cursor rule in metamask-design-system — MetaMask/metamask-design-system#1462

Type of Change

• New skill
• Skill improvement/update
• Bug fix
• Documentation update
• Other (please describe):

Skill Details (if adding a new skill)

Provider Name: MetaMask
Skill Name: content-guidelines
Brief Description: MetaMask content design rules for user-facing strings — sentence case, punctuation, voice, tone, and terminology. Use when writing or reviewing copy in component stories, constants, locales, or any user-facing string.

Checklist

• I have read the CONTRIBUTING.md guidelines
• My skill follows the SKILL_TEMPLATE.md format
• I have tested this skill with an AI agent
• My skill does not contain any secrets, private keys, or sensitive data
• I have added appropriate documentation
• My changes don't break existing skills

Testing

The rule content was drafted against the MetaMask Content Design Style Guide and validated through use in metamask-mobile and metamask-design-system PRs. Full AI agent testing pending.

Additional Context

The skill lives under domains/ui/ per the CODEOWNERS assignment for @MetaMask/design-system-engineers, which owns that domain. The mobile-specific content is in a repos/metamask-mobile.md overlay.

[georgewrmarshall]

This is a strong first version of the Mobile content guidance. I left two suggestions to make product locale content the primary use case and keep changing component guidance in MMDS documentation, where Storybook MCP can expose it. The shared capitalization, punctuation, voice, tone, terminology, and formatting rules look useful.

[georgewrmarshall]

suggestion: Could we lead with locale files here and in the skill description? Product strings are the main user-facing source. Stories, constants, and README examples should follow the same rules, but they are secondary.

For example: Writing or reviewing user-facing product strings in locale files, then checking the same content standards in stories, constants, and README examples.

[georgewrmarshall]

suggestion: Could we remove this entire per-component section and move each rule into the relevant component documentation in MetaMask/metamask-design-system?

The agentic design system strategy says skills should stop carrying copied component knowledge because it becomes stale after releases. Component-specific content guidance should live beside the component, ship with MMDS, and reach agents through Storybook MCP.

A practical next step would be to ask Cursor to create a MetaMask Design System PR that:

1. Removes the per-component section from .cursor/rules/content-guidelines.mdc.
2. Adds a Content guidelines section to each relevant React and React Native component README.
3. Moves only current rules to current MMDS components. It should not migrate legacy Mobile names.
4. Updates component stories and sample copy where they conflict with the new README guidance.
5. Confirms the README content appears in the Storybook documentation exposed through Storybook MCP.

This skill can then keep the stable rules for capitalization, punctuation, voice, tone, terminology, dates, and numbers. It should route agents to MMDS for component-specific guidance.

[georgewrmarshall]

LGTM 🔥 Both earlier suggestions are addressed. Locale files are now the primary use case, and changing component guidance has moved out of the skill. The related MMDS PR provides the component README path needed for the longer-term Storybook MCP model. This looks good as a first Mobile version.