[AVC] Guideline update ingestion pipeline

[tjprescott]

We need to implement a system that automatically detects and ingests changes to SDK guidelines from the azure-sdk repository in an incremental and reliable way.

The system should identify guideline updates by comparing Git commits, reprocess only the affected content, and propagate those changes to storage and search so downstream consumers stay up to date.

By tracking source metadata and content hashes, this approach should minimize unnecessary re-ingestion, correctly handle updates and deletions, and support safe operation via dry runs and forced resyncs.

The goal is to establish a low-noise, maintainable pipeline that keeps guidelines synchronized with their source of truth over time.