Support separate pattern files for renames vs content replacements

## Summary

Currently, the same patterns are applied to both file content replacements and file path renames (when using `--full`). There is no way to specify different patterns for each operation.

## Details

There is a TODO at line 1638 of `repren.py`:

```
# Separate patterns file for renames and replacements
```

Use cases where separate patterns would be useful:
- Renaming files according to one convention while replacing content with a different pattern
- Using regex features in content patterns that don't make sense for file paths
- Applying broader patterns to content but more conservative patterns to filenames (to avoid accidental directory restructuring)

## Suggestion

Add an optional `--rename-patterns` flag (or `--content-patterns`) that allows specifying a separate patterns file for renames vs content replacements. When not specified, the existing behavior (same patterns for both) would be preserved.