Issue 007: Prompt playbook structure

[cody257]

Summary

• Adds concise prompt playbook markdown for the Haggly agent harness.
• Adds a server-side prompt assembler that selects playbooks from mode/context/latest message and returns system/user prompt parts.
• Adds tests for seller marketplace, buyer dealership, buyer private-seller, and assembled prompt output.

Test Plan

• npm run lint
• npm test
• npm run test:e2e
• npm run build

Notes

• No API calls, model provider, keys, auth, database, or deployment changes included.
• Old research/script docs remain as reference material; new playbooks are guidance, not canned scripts.