Add headless example for commons_harvest substrate

[morty649]

This PR adds a minimal headless example demonstrating how to interact with the
commons_harvest__open substrate programmatically without a graphical display.

Motivation:

• Existing tutorial examples rely on pygame and require a GUI
• New users running Melting Pot in headless environments (servers, CI, or
  GitHub Codespaces) lack a simple reference for stepping environments
• TensorFlow/XLA initialization warnings and sparse rewards can be confusing
  for first-time users

What this adds:

• A lightweight, headless example using substrate.build
• Random-action stepping to demonstrate observations and rewards
• Inline documentation explaining expected warnings and sparse rewards

Notes:

• Rewards are intentionally sparse and may remain zero for many steps
• TensorFlow/XLA initialization warnings may appear in headless environments
  and are harmless

No core logic is modified.

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