[H-5747] Set up dev harness for Agentic Workflows

[lunelson]

🌟 What is the purpose of this PR?

Initial implementation of Mastra-based Named Entity Recognition (NER)
system, establishing foundation for migrating AI inference capabilities
from hash-ai-worker-ts to structured Mastra architecture.

🔗 Related links

• Detailed architecture mapping:
  apps/hash-ai-agent/docs/mastra-migration-plan.md
• Phase 2 roadmap: apps/hash-ai-agent/docs/NEXT_STEPS.md

🔍 What does this change?

• Created entity type schemas with Zod validation
  (src/mastra/types/entities.ts)
• Ported entity type dereferencing utility from hash-ai-worker-ts
  (src/mastra/shared/)
• Implemented baseline NER agent using Google Gemini 2.0 Flash via
  OpenRouter
• Created entity recall scorer with weighted penalty evaluation
• Ported 4 stable test cases with ground truth data
• Registered new agent and scorer in Mastra instance

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

• does not modify any publishable blocks or libraries, or modifications do
  not need publishing

📜 Does this require a change to the docs?

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

• do not affect the execution graph

⚠️ Known issues

None. This is Phase 1 foundation only - does not yet include claims
extraction or full entity proposal pipeline.

🐾 Next steps

Phase 2: Implement claims-based extraction (claim extraction agent →
entity proposal agent → three-step workflow). See docs/NEXT_STEPS.md.

🛡 What tests cover this?

• Smoke test: src/mastra/evals/test-entity-extraction.ts
• Full evaluation: src/mastra/evals/run-entity-extraction-eval.ts
• Entity recall scorer with 4 test cases covering edge cases

❓ How to test this?

1. Checkout the branch
2. Ensure OPENROUTER_API_KEY is set
3. Run: cd apps/hash-ai-agent && pnpm tsx
   src/mastra/evals/test-entity-extraction.ts
4. Verify agent extracts entities from test case