Create GitHub Copilot agent skill for upstream synchronization

ArcGIS Enterprise environments often require maintaining parity between public reference architectures (e.g., Esri GitHub repositories for automated deployment) and hardened private corporate mirrors. In particular, this is required by the [ArcGIS Enterprise upgrade procedure](https://github.com/Esri/arcgis-gitops/wiki/ArcGIS-Enterprise-Upgrades).

GitHub Copilot agent skill **arcgis-sync-upstream** should support:

- Upstream Merging & Conflict Resolution: Pulls upstream changes from public [esri/arcgis-gitops](https://github.com/esri/arcgis-gitops) repository into private downstream repositories containing organization-specific modifications.
- Custom Patch Preservation: Automatically identifies local overlays and preserves them during upstream merges.
- Drift Detection: Compares active infrastructure automation code against deployed ArcGIS Enterprise baseline standards to flag unauthorized or uncommitted modifications.

Agent Execution Pattern: The agent performs a three-way merge strategy, utilizing semantic diffing to ensure that upstream bug fixes are integrated without overwriting internal enterprise configuration parameters.

Start with the ArcGIS Enterprise upgrade procedure and the esri/arcgis-gitops repository referenced in the issue. Define how the arcgis-sync-upstream skill performs a three-way merge, preserves local overlays, and detects drift against the deployed baseline. Done means upstream fixes are integrated without overwriting enterprise configuration and unauthorized modifications are reported.

Written by the indexing model from the issue text.