feat: add teaching asset prompts

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

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+# Phase 6 Plan: Teaching Assets
+
+**Status:** Ready for execution
+**Phase Goal:** Create source-backed teaching asset prompt candidates.
+
+## Requirements Covered
+
+- ASSET-01: System can generate prompt candidates for visual teaching assets.
+- ASSET-02: Generated assets store source concept, evidence, prompt, model
+  config, and review state.
+- ASSET-03: Image model configuration verifies the actual OpenAI model
+  identifier before production calls.
+
+## Tasks
+
+### 1. Add teaching asset package
+
+- Define asset prompt candidate, asset type, review state, and snapshot types.
+- Add in-memory store and service.
+
+### 2. Generate prompts from ontology evidence
+
+- Select source-backed ontology concept by concept id.
+- Generate prompt candidate for diagram, lesson slice, worksheet, or interview
+  card.
+- Reject prompt generation when concept evidence is missing.
+
+### 3. Add HTTP endpoints
+
+- `POST /api/v1/teaching-assets/prompts`
+- `GET /api/v1/teaching-assets`
+
+### 4. Add tests and verification
+
+- Test prompt candidates keep concept and source evidence.
+- Test model verification guard remains true.
+- Test HTTP material-ingest-to-asset-prompt flow.
+- Run Go tests, OpenSpec validation, line-count check, and smoke.
+
+## Out of Scope
+
+- Real image generation.
+- Slide deck creation.
+- Asset publishing.