Add custom model integration plugin interface

[jzonthemtn]

Summary

There is no plugin point for users to bring their own ML models alongside the built-in detectors. Teams with domain-specific NER models must fork Phileas to integrate them.

Proposal

Add a model-provider interface:

• Define a standard SPI/interface for custom entity detectors
• Allow registration of custom detectors alongside built-in ones
• Support model loading from local paths or remote endpoints
• Merge results from custom and built-in detectors using configurable precedence

This lets teams add domain-specific detection without forking.

Acceptance Criteria

• Plugin interface allows users to register custom NLP models for entity detection
• Plugins conform to a documented interface (input: text, output: entity spans + types + scores)
• Phileas routes detection through both built-in and plugin models per policy configuration
• At least one example plugin is provided
• Plugin API documented