ai-agents: add per-component accessibility schema to /r/<name>.json

[bntvllnt]

Problem

Agents generating UI from VLLNT components ship a11y bugs because they don't know the keyboard model, ARIA roles, or focus expectations of each component.

Acceptance criteria

• Add a11y object to /r/<name>.json:

  a11y: {
    role?: string;                  // e.g., 'dialog', 'menu', 'tablist'
    keyboard?: Array<{ keys: string; action: string }>;  // e.g., { keys: 'Enter / Space', action: 'activate' }
    aria?: string[];                // attributes consumers must set: 'aria-label', etc.
    focusManagement?: 'auto' | 'manual';
    notes?: string;
  }

• Source: meta.json per component or extracted from Radix primitive metadata where applicable.
• Component page renders an "Accessibility" section (collapsed by default).
• /llms-full.txt includes a11y schema (ai-agents: serve /llms-full.txt with full docs + component summaries #236).
• MCP get_component returns a11y (ai-agents: ship @vllnt/mcp — MCP server (search/get/install components) #246).
• CI guard: components with Dialog, Menu, Tabs, Combobox patterns must declare a11y.

References

• ARIA Authoring Practices: https://www.w3.org/WAI/ARIA/apg/patterns/