feat(engine): context-based annotations via LoadContext

[martsokha]

Summary

Support loading reusable annotation sets as context entries via LoadContext nodes. This allows annotation templates (e.g., "always treat these regions as PII in this document template") that apply across multiple pipeline runs.

Split from #126 which covers upload-time annotations. This issue depends on context variant type wiring (#129).

Design

• Define an AnnotationContext context entry type containing Vec<Annotation>
• LoadContextOp recognizes annotation context entries and attaches them to the envelope
• During detection, annotation exclusions filter detected entities and annotation inclusions are added as entities

Tasks

• Define AnnotationContext context entry type
• Wire into LoadContextOp
• Apply exclusions/inclusions during detection phase
• Integration tests

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