feat: add teaching asset prompts

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

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+# Phase 6 Verification
+
+## Verdict
+
+PASS
+
+## Requirement Coverage
+
+- ASSET-01: PASS. Teaching asset prompt candidates can be generated for
+  ontology concepts.
+- ASSET-02: PASS. Prompt candidates store source concept, evidence, prompt,
+  model key, and review state.
+- ASSET-03: PASS. Prompt candidates carry
+  `requires_model_id_verification=true`, so production image generation remains
+  blocked until the provider model identifier is verified.
+
+## Evidence
+
+- `go test ./...` passed.
+- `openspec validate bootstrap-job-tutor-platform --strict` passed.
+- Live material-to-asset-prompt smoke passed.
+- Go source line-count check passed.
+
+## Residual Risk
+
+`gpt-image-v2` is currently treated as the product configuration key, not a
+confirmed provider model id. A future production generation phase must verify
+the actual OpenAI model identifier against current official docs before making
+real calls.