feat: Phase-4 manifest builder with enhanced file classification

[ZeroSumQuant]

Summary

Implements Phase-4 Task A - manifest builder for file classification and tracking.

Changes

File Classification

Added comprehensive file classification in classify_file():

• binary: Images, archives, compiled files (.png, .zip, .pyc, etc.)
• template: HTML/XML templates (.html, .jinja2, .mustache, etc.)
• notebook: Jupyter notebooks (.ipynb)
• test: Python files with pytest/unittest imports
• script: Python files with shebang or __name__ == '__main__'
• module: Other Python files
• data: CSV, JSON, Parquet, HDF5 files
• documentation: Markdown, RST, LICENSE, README files
• configuration: Project config files (pyproject.toml, .gitignore, etc.)
• other: Everything else

Manifest Features

• Records file size, suffix, and executable permissions
• Flags data files >20MB as "oversize"
• Excludes common build/cache directories
• Saves manifest to .cake/manifest.json
• Provides summary counts by category

Tests

Added comprehensive test suite in test_phase4_manifest.py:

• Tests for each file classification type
• Tests for oversize file detection
• Tests for directory exclusion logic
• Tests for manifest structure and saving

Next Steps

After merge, implement Phase-4 Task B - the organizer that uses this manifest to reorganize files into proper directories.

Testing

All tests pass:

pytest tests/test_phase4_manifest.py -v
# 17 passed