docs(checkpoint): added readme for mental-models project transitions

[wendev27]

🎯 Summary

This PR initializes the new mental model document describing how the projects in this repository evolve from tool-focused experiments into production-oriented systems.

Changes included

• Added mental-model-project-transitions.md.
• Documented the relationship between:
  • DevSecOps-Learning-Lab
  • Secured-Payment-Paymongo-Practice
  • DevOps-War-Room
  • Sentinel-Core

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🧠 Motivation

The purpose of this PR is to establish the foundation for observability while documenting how each project builds upon previous concepts.

The goal is not simply to learn individual technologies, but to understand how security, infrastructure, monitoring, and distributed systems fit together.

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📚 Lessons learned

• Logging is one of the foundations of observability.
• Health checks expose application state to external systems.
• Docker containers provide a reproducible runtime environment.
• Monitoring becomes more important as systems grow in complexity.
• The same concepts reappear across different projects, but in more advanced forms.

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🚀 Next steps

Upcoming work for this lab includes:

• Prometheus integration
• Metrics collection
• Grafana dashboards
• Observability workflows
• Monitoring containerized services