test: add context strategy goldens

[Activer007]

Summary

• add golden coverage for raw_transcript, natural_context, context_capsule, and hybrid_recent context rendering
• neutralize raw transcript's prompt-visible title while preserving its legacy role-delimited shape
• update the strategy guide to describe raw_transcript as a neutral-title rollback strategy

Tests

• go test ./internal/promptcompat
• go test ./internal/httpapi/openai
• ./scripts/lint.sh
• ./tests/scripts/check-refactor-line-gate.sh
• ./tests/scripts/run-unit-all.sh
• npm run build --prefix webui