ModelRegistry's public constructor became private in pi-coding-agent 0.64.
Direct `new ModelRegistry(...)` calls no longer compile. Switch the five
production sites to the factory (`ModelRegistry.create`) and update the
four test modules that mocked the class as a constructor to now mock it
as an object with `create` and `inMemory` static methods.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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