Add energy-based agent example (needs-based behavior)

I would like to contribute a new example demonstrating
needs-based agent behavior using an energy model.

The example will include:
- Agents losing energy over time
- Decision-making based on energy levels (move vs rest)
- Simple, clear structure for educational purposes

This aligns with the behavioral modeling direction mentioned
in the GSoC ideas (needs-based behavior exploration).

Please let me know if this approach is acceptable.

No file, test, or entry point is named. Start by reviewing the repository's existing example structure and the referenced GSoC behavioral-modeling direction; done means a clear, runnable energy-based agent example that demonstrates energy loss and move-versus-rest decisions.

Written by the indexing model from the issue text.