Add keyrotate

[sophie4869]

Adding keyrotate to the Secret Management section.

What it is: open-source (MIT) bash CLI for rotating and syncing secrets across the places they physically live — Vercel env, Google Cloud Run env, GCP Secret Manager, Koyeb, GitHub Actions secrets (across N repos), MongoDB Atlas user passwords, remote hosts over SSH, and local .env files — from a single values-free JSON config per project.

Why it's a fit for this section: existing entries (Vault, SOPS, Infisical, Lade) focus on encrypted storage + runtime fetch. keyrotate solves a complementary problem: rotation + synchronization across ~8 different sinks for real personal / small-team infrastructure where one MONGODB_URI lives in 4 places and an e2e test token lives across 8 repos' Actions secrets. It also adds cross-project propagation so a JWT signing key owned by one auth service can automatically land in all its downstream verifiers.

Meets contribution guidelines:

• MIT licensed, hosted at https://github.com/sophie4869/keyrotate
• Format: [NAME](LINK) - DESCRIPTION. (79-char description, ends with period, no trailing whitespace)
• Added at end of the Secret Management section (per the "add at bottom" rule)
• Single commit, imperative title (Add keyrotate to Secret Management)

Happy to adjust wording, placement, or trim further if useful.