Add AGENTS.md with Cursor Cloud development environment setup

[riteshverma]

Summary

Adds AGENTS.md with Cursor Cloud specific instructions for setting up and running the S18Share development environment.

What's included

• Development server startup commands
• Key API endpoints reference
• Environment configuration notes (.env, Gemini API key, Ollama)
• Test execution instructions (unittest-based tests in tests/)
• Known gotchas (Yahoo Finance MCP server, auto-created config files, persistent state directories)

Environment Setup Verified

Check	Status
uv sync (dependency install)	Passed
python -m compileall (compile check)	Passed
API startup (uvicorn api:app)	Passed
/health endpoint	Passed
/docs (Swagger UI)	Passed
/metrics/prometheus	Passed
/metrics/dashboard	Passed
Chat session creation	Passed
Agent run creation	Passed
MCP servers (browser, rag, sandbox, mockehr)	Connected
tests/audit_features.py	2/2 passed
tests/test_supabase_auth.py	7/8 passed (1 pre-existing flaky test)