feat: add Agent Interfaces components

[starc007]

What changed

• adds 16 production-ready Agent Interfaces components for reasoning, messages, streaming responses, tools, approvals, citations, code, diffs, tasks, prompts, image generation, and workspace navigation
• adds a complete Chat App composition example demonstrating the components together
• adds component guides, previews, registry packaging, navigation, SEO metadata, and scheduled launch ordering
• hardens keyboard behavior, focus management, screen-reader announcements, reduced-motion handling, streaming lifecycles, and optimistic sidebar moves

Why

AI product interfaces need reusable primitives that can render progressive agent work without each application rebuilding the same interaction and accessibility behavior.

Impact

Developers can install individual Agent Interfaces through the beUI registry and compose them with AI SDK message and tool state. The Chat App example demonstrates a complete integration without becoming a required runtime abstraction.

Validation

• bun run check
• bun test — 170 passed, 0 failed
• registry validation — 72 components