chore: add Mix agent skill

[leoafarias]

Description

Adds the Mix framework skill under .agents/skills/mix so agents working in Remix can use the Mix repository's current guidance for architecture, styling APIs, variants, code generation, testing, and development workflows.

The skill content is copied verbatim from conceptadev/mix at 726d85e, and .claude/skills points to the canonical .agents/skills directory for discovery.

Validation

• Skills setup check — passed with one installed mix skill and valid Claude discovery.
• Source comparison against conceptadev/mix/skills/mix — byte-for-byte match.
• git diff --check — passed.

Related Issues

None.

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Checklist

• Tests are not required because this PR changes only repository-local agent instructions.
• Relevant usage and development documentation is included in the skill.
• I am prepared to follow up on review comments in a timely manner.

Breaking Change

Does this PR require users of the package to manually update their code?

• Yes, this is a breaking change.
• No, this is not a breaking change.