Initialize project folder structure

[ajharris]

Create the root repo structure as defined in README.

Tasks:

• Add folders: src/, config/, notebooks/, docs/, data/
• Add subfolders under src/: ingest/, preprocess/, features/, train/, eval/, utils/
• Add placeholder init.py to each Python package folder
• Add/update .gitignore to exclude data/, large outputs, secrets
• Add LICENSE (MIT), CODE_OF_CONDUCT.md if desired

Acceptance criteria:

• Repo mirrors README structure
• Imports work from src without path hacks

Dependencies: none