Add shekel - runtime budget guardrails for AI agents

[arieradle]

shekel is a Python library providing runtime budget guardrails for AI agents.

Problem: AI agents run up unexpected costs via retries, tool loops, and runaway LLM calls. shekel sets hard spending limits.

Usage:

from shekel import budget
with budget(max_usd=5.00):
    run_my_agent()

Supports: OpenAI, Anthropic, Gemini, LangChain, LangGraph, CrewAI, AutoGen, LlamaIndex, MCP, and more.

[awesomekoder]

Thanks for the interest! This PR adds README links but doesn't include runnable code in its own folder. For this repo, we need self-contained tutorials with the actual implementation included.