Add Terradev + Weights & Biases example integration

[theoddden]

This PR adds a new example under examples/terradev/ showing how to use Terradev's cross-cloud GPU cost optimization together with Weights & Biases.

What is included:

• terradev_wandb_example.py - fetches a live GPU quote from Terradev (or uses a sample quote in demo mode), initializes a W&B run with the provider/region/cost as config, and logs a simulated training loop with loss, accuracy, GPU utilization, and cumulative cost.
• requirements.txt - lists wandb (and Terradev is documented as optional for live quotes).
• README.md - setup and usage instructions.
• A new section in examples/README.md cataloging the example.

The script works out of the box in demo mode without any cloud credentials. Passing --live fetches real quotes from the Terradev CLI.