feat: add teaching asset prompts

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

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+# Phase 6 Summary
+
+**Status:** Complete
+**Completed:** 2026-04-26
+
+## Delivered
+
+- Added `internal/teachingassets` for prompt candidates and snapshots.
+- Added image model config key `TUTOR_IMAGE_MODEL_KEY`, defaulting to
+  `gpt-image-v2`.
+- Added workflow contract structs for `OntologyGap` and
+  `TeachingAssetPrompt`.
+- Added prompt generation from source-backed ontology concepts.
+- Added model-id verification guard on every prompt candidate.
+- Added HTTP endpoints:
+  - `POST /api/v1/teaching-assets/prompts`
+  - `GET /api/v1/teaching-assets`
+- Added service and HTTP tests.
+
+## Verification
+
+```powershell
+gofmt -w cmd internal
+go test ./...
+openspec validate bootstrap-job-tutor-platform --strict
+```
+
+Additional smoke check:
+
+- Material ingestion followed by teaching asset prompt generation returned a
+  source-backed prompt with `requires_model_id_verification=true`.
+
+## Deferred
+
+- Real image generation calls.
+- Provider SDK integration.
+- Binary asset storage.
+- Slide/PPT export.