Support merging multiple registries in a single call

[varunreddy]

Summary

Allow the CLI to accept multiple --registry flags and merge them before retrieval, so users can combine domain registries without maintaining a single monolithic file.

Acceptance Criteria

• skillmesh retrieve and skillmesh emit accept multiple --registry arguments
• Registries are merged with duplicate ID detection (error on conflict)
• Add unit tests for multi-registry merging
• Update CLI help text and README
• Example: skillmesh retrieve --registry devops.registry.yaml --registry cloud-gcp.registry.yaml --query "deploy to GKE" --top-k 3