Add metadata support to categorizer (v0.8.0) (#15)

This update allows categorizers to add custom metadata to transactions through
the categorizer return dict. This enables better transaction tracking,
analysis, and compliance.

Changes:
- Add 'metadata' field to CategorizerReturn type
- Update both BancoChileImporter and BancoChileCreditImporter to merge
  custom metadata from categorizer results
- Add comprehensive tests for metadata functionality
- Update CLAUDE.md documentation with metadata examples
- Bump version to 0.8.0

Example usage:
```python
def categorizer(date, payee, narration, amount, metadata):
    if "AMAZON" in payee.upper():
        return {
            "category": "Expenses:Shopping:Online",
            "metadata": {
                "merchant_category": "online",
                "tax_deductible": False,
                "reviewed": True,
            }
        }
    return None
```

All 80 tests pass successfully.