feat: add teaching asset prompts

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

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+# Phase 6 Context: Teaching Assets
+
+**Status:** Ready for execution
+**Started:** 2026-04-26
+
+## Goal
+
+Generate reviewable teaching asset prompt candidates from ontology concepts.
+
+## Inputs
+
+- OpenSpec generated study asset lineage requirement.
+- `docs/planning/WORKFLOW_CONTRACTS.md` TeachingAssetPrompt contract.
+- Phase 5 ontology candidates and source evidence.
+- PRD requirement to verify actual OpenAI model identifier before production
+  image calls.
+
+## Decisions
+
+- Generate prompt candidates only; do not call an image provider in this phase.
+- Default product model key remains `gpt-image-v2`.
+- Keep `requires_model_id_verification=true` until a future production
+  integration verifies the actual provider model identifier.
+- Persist prompt lineage with concept, evidence, model key, and review state.
+
+## Boundaries
+
+In scope:
+
+- Teaching asset prompt candidate service.
+- Asset prompt and snapshot APIs.
+- Model verification guard represented in output.
+
+Out of scope:
+
+- Real image generation calls.
+- Binary asset storage.
+- PPT export.
+- Provider-specific OpenAI SDK integration.