feat: add safe educational MCP prompts

[adity982]

Closes #14

Summary

• advertise the MCP prompts capability and implement prompts/list and prompts/get
• add three reusable educational workflows for ABCDE observations, appointment preparation, and SNOMED explanations
• include the non-diagnostic disclaimer in every rendered prompt and reject missing terminology codes
• document the prompt surface and cover the mock server end to end

Validation

• npm test — build passed; 80/81 tests passed. The one failure is the existing network-sensitive no API key reaches stderr test, which timed out without server logs; it is unrelated to prompts.
• npm run build — passed
• node --test tests/prompts.test.mjs — 2 passed
• git diff --check — passed

This is an AI-assisted contribution. Mock mode is used; no credentials, patient photos, or network calls are required by the new tests.