Add @keelstack/guard to Runtime → Tool section

[siddhant-jain-18]

Summary

• Add @keelstack/guard to the Runtime → Tool section.
• MIT license, zero‑dependency TypeScript runtime guardrails for AI agents.

Section

• ## Runtime → ### Tool (at the bottom of the list)

Checklist

• I placed the entry in the most appropriate category
• I verified the project or resource link works
• The GitHub repository is not archived
• The project has recent activity (commits, tests, coverage)
• I followed the formatting style used in the README (badges: stars, contributors, last‑commit)
• I placed the new item at the bottom of the section

Notes

• This project adds production‑grade safety primitives for AI agents (idempotency, cost budgets, risk gates), complementing existing SidClaw (approval layer) and WritBase (task management).
• The package is already used in several framework examples (LangGraph, Vercel AI SDK, Mastra, OpenAI Agents SDK).