feat: add Angular & Sanity integration guide

[kmelve]

Angular & Sanity Integration Guide

Adds a comprehensive Angular integration guide to the sanity-best-practices skill, matching the format and style of existing framework guides (Next.js, Nuxt, SvelteKit, Astro, Remix).

What's included

references/angular.md (~10KB, 10 sections):

1. Setup & Configuration — Official template (sanity-template-angular-clean), dependencies, environment config
2. Client Setup — Injectable SanityService wrapping @sanity/client + @sanity/image-url
3. Data Fetching — Three patterns with comparison table: resource (recommended), rxResource, toSignal
4. Routing — Lazy-loaded routes with withComponentInputBinding()
5. Portable Text — @portabletext/to-html pipe + community library mention
6. Image Optimization — SanityImagePipe + NgOptimizedImage integration
7. Modern Angular 19+ — Standalone components, signals, @defer, zoneless, inject()
8. SSR & Prerendering — Built-in SSR, hydration, getPrerenderParams
9. Visual Editing — Honest about no official support; basic preview pattern; community library
10. Error Handling — Common errors table

SKILL.md — Added Angular to the integration guides list.

Research sources

• Official template: sanity-io/sanity-template-angular-clean (Angular 19.2, zoneless, standalone, Resource API)
• Angular 19 docs (signals, resource API, SSR, @defer)
• Community: @limitless-angular/sanity for Portable Text components
• Existing agent-toolkit guides for format/style reference

Key decisions

• Recommended resource API as primary data fetching pattern (Angular 19+)
• Honest about Visual Editing gap (no official Sanity support for Angular)
• Angular-idiomatic patterns throughout (services, pipes, DI, standalone components)
• Cross-references groq.md, schema.md, image.md, portable-text.md