Add @keelstack/guard to Developer Tools section

[siddhant-jain-18]

Summary

• Added @keelstack/guard to the Developer Tools section.
• MIT-licensed, zero‑dependency TypeScript runtime guardrails for AI agents: idempotency (replay on retry), per‑user LLM cost budgets, risk gates (allow/log/warn/block) for irreversible actions.
• Works with LangGraph, Vercel AI SDK, Mastra, OpenAI Agents SDK, or any raw async function.

Section

• Developer Tools (under the main README)

Checklist

• I placed the entry in the most appropriate category
• I verified the project or resource link works
• If this is a GitHub repository, it is not archived
• If this is a Community Project, it has recent activity and is not stale
• I followed the formatting style used in the README
• I updated related badges, descriptions, or section placement if needed

Notes

• The entry is placed at the bottom of the Developer Tools section as required by the contribution guidelines.
• The repository is active (recent commits, 37 passing tests, 98% coverage).
• The description uses the same badge format (![GitHub Repo stars]) as existing entries.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.