[Docs] Update AGENTS.md and synchronize CLAUDE.md with Sistent design guidance

Current Behavior

The layer5io/layer5 repository already has an AGENTS.md, but its UI styling guidance still refers to "Tailwind/CSS modules" without accounting for the repository's dependency on Sistent.

This can leave contributors and AI coding agents without a clear reference to the shared Sistent design contract when making UI changes.

Desired Situation

Update the existing AGENTS.md to reference Sistent's design contract for UI guidance.

The updated guidance should:

• Reference @sistent/sistent for shared UI components and styling.
• Point contributors and AI coding agents to sistent/DESIGN.md for the design contract, tokens, and UI conventions.
• Replace or update the outdated "Tailwind/CSS modules" styling guidance where appropriate.
• Avoid duplicating the contents of sistent/DESIGN.md inside this repository.

The goal is to keep the repository's agent/contributor guidance aligned with Sistent while maintaining a single source of truth for the design system.

Contributor Resources and Handbook

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