Add multi-registry documentation

[Copilot]

• Research multi-registry feature implementation
• Understand registry_manifest.yaml structure and dependencies
• Understand imports section functionality
• Understand --include-unreferenced flag behavior
• Review test data and examples
• Create docs/multi-registry.md file with complete documentation
• Verify documentation accuracy with actual code
• Remove deprecated search command from examples
• Add note about current dependency limitation
• Fix example to include auction.name attribute
• Remove misleading best practice about testing both modes
• Remove Use Case 3 (Testing Registries) section
• All requested changes complete

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.9%. Comparing base (720c984) to head (b64704f).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1190   +/-   ##
=====================================
  Coverage   79.9%   79.9%           
=====================================
  Files        109     109           
  Lines       8528    8528           
=====================================
  Hits        6821    6821           
  Misses      1707    1707           

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