Add accessibility report cookbook recipe

Add a new cookbook recipe that generates WCAG accessibility reports using
the Playwright MCP server. Includes streaming output, structured report
formatting, and optional Playwright test generation.

- C#, TypeScript, Python, and Go implementations
- Markdown documentation for each language
- Updated cookbook.yml, copilot-sdk README, and package.json

cookbook/cookbook.yml

@@ -61,3 +61,11 @@ cookbooks:
           - sessions
           - persistence
           - state-management
+      - id: accessibility-report
+        name: Accessibility Report
+        description: Generate WCAG accessibility reports using the Playwright MCP server
+        tags:
+          - accessibility
+          - playwright
+          - mcp
+          - wcag