[surface-ghn] T-046 KB loader

[zigrivers]

Summary

• Add a gray-matter markdown knowledge-base loader in @surface/core
• Extend KnowledgeEntry metadata with category, citations, freshness, relevance fields, and deep guidance
• Turn US-070 acceptance coverage on for inspectable cited KB entries

Validation

• pnpm exec vitest run packages/core/src/knowledge-source.test.ts tests/acceptance/epic-e7-e8-integrations-kb.test.ts
• pnpm run check
• pnpm run test:evals
• git diff --check