Add Registry Ping Command

[toddysm]

Add a simple ping command to test registry connectivity via GET /v2/.

Acceptance Criteria

• Implement regshape ping --registry <registry> command
• Show registry reachability and basic info
• Handle authentication as needed
• Provide clear success/failure output
• Add appropriate exit codes
• Add tests

Files to Create/Modify

• src/regshape/cli/ping.py or add to existing module
• Update src/regshape/cli/main.py

References

• OCI Distribution Spec: Base endpoint /v2/