Add AWS AppSync Event API instruction and regenerate docs

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Summary

Adds a new public instruction file for AWS AppSync Event API development:

• instructions/aws-appsync.instructions.md

This guide is designed for production use and focuses on APPSYNC_JS event handlers (onPublish, onSubscribe) with strict runtime-safe patterns.

What’s Included

• Runtime restrictions for APPSYNC_JS (unsupported language features, control-flow limits, no async/promise usage)
• Built-in utilities and runtime utility usage (util.*, runtime.earlyReturn)
• Built-in module guidance for:
  • @aws-appsync/utils/dynamodb
  • @aws-appsync/utils/rds
• Data source mapping and usage guidance for Event API:
  • Lambda, DynamoDB, RDS, EventBridge, HTTP, OpenSearch, Bedrock
• Data source setup and IAM hardening:
  • least privilege policies
  • trust principal appsync.amazonaws.com
  • aws:SourceAccount and aws:SourceArn restrictions
• Handler flow guidance:
  • when to return transformed events
  • datasource request/response structure
• Pipeline data-flow guidance:
  • use ctx.prev.result for sequential step chaining
  • use ctx.stash for cross-step shared metadata/state
• Batch operations coverage:
  • DynamoDB batch and transaction patterns (batch*, transact*)
  • Lambda batching strategy for Event API (Invoke with list payloads)
  • explicit note that Event API handlers do not use BatchInvoke request objects
• Error handling and authorization flow recommendations
• Observability and production-readiness checklist

Why

AWS AppSync Event API behavior is specific and easy to misuse with generic GraphQL/AppSync assumptions. This instruction file provides an opinionated, safe baseline for public-facing event systems, reducing runtime errors and improving consistency/security.

Notes

• File follows repository naming and frontmatter conventions.
• PR targets staged branch per repository contribution guidance.