feat: add feature-flags skill with MetaMask Mobile overlay

[NicolasMassart]

Description

Adds feature-flags in the platform domain: shared skill.md, MetaMask Mobile overlay, and CHANGELOG. Marked base: true so it installs with the default base set.

Why a shared skill plus a repo overlay. skill.md is the workflow every consumer shares. repos/metamask-mobile.md is the Mobile wiring. A Mobile-only skill would be fewer files today; the next consumer would then copy or fork it. Overlay merge is the existing pattern in this repo.

Why base: true. Engineers who never select the platform domain still get this skill on postinstall, same as the other base skills. Relying on --include or SKILLS_DOMAINS=platform would miss most clones. Cost: always-on description tokens. --exclude / SKILLS_EXCLUDE still skips it.

Fixes MCWP-827.

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: feature-flags
Brief Description: Version-gated remote feature flags, with a MetaMask Mobile overlay for the canonical helper and selectors.

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

• Confirm yarn audit:skills accepts the new skill frontmatter
• Install the skill and confirm it is selected when adding or reviewing feature flags in MetaMask Mobile
• Confirm base: true still installs the skill when its domain is filtered out
• Confirm --exclude / SKILLS_EXCLUDE still skips it when explicitly excluded

Proof of testing the skill

See the generated skill test report: skill-test-feature-flags.local.md

Additional Context

MCWP-827

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