feat: add teaching asset prompts

.planning/phases/006-teaching-assets/006-RESEARCH.md

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+# Phase 6 Research: Teaching Assets
+
+## Findings
+
+The product needs lineage before generation. A prompt candidate that carries
+concept, source evidence, model key, and review state is already useful because
+it can be reviewed before spending image-generation cost or publishing content.
+
+## Recommended Shape
+
+- `internal/teachingassets` owns asset prompt candidates.
+- Use ontology snapshot as the source for concept/evidence lookup.
+- Generate prompts deterministically for MVP asset types.
+- Store `requires_model_id_verification=true` so production image generation is
+  blocked until the provider model identifier is verified.
+
+## Risks
+
+- Calling image generation before model ID verification would violate PRD.
+- Prompt candidates without evidence would weaken provenance.
+- Real slide/PPT generation should be a later phase.