truss init CLI to accept a positional project name argument

[pat-baseten]

🚀 What

truss init CLI to accept a positional project name argument following conventions established by tools like npm init, cargo new, and django-admin startproject.

Before:

• truss train init created a directory named truss-train-init
• truss train init my-project did nothing useful (argument was ignored)

After:

• truss train init creates a hello-world/ directory with a working MNIST
  example
• truss train init my-project creates a my-project/ directory
• Provides clear "Next steps" guidance after initialization
• Errors gracefully if the directory already exists

💻 How

1. CLI argument parsing (truss/cli/train_commands.py):

• Added PROJECT_NAME as a positional argument with default hello-world
• Kept --target-directory option for --examples mode

2. Updated default template (truss/templates/train/):

• Added train.py with PyTorch MNIST classifier training code

🔬 Testing

• All CLI tests pass
• Manual testing verified:
  • truss train init creates hello-world/ with all 4 files
  • truss train init my-project creates my-project/ correctly