fix: add Yarn v1 global cache cleanup

[khisby]

Summary

• Add cleanup for Yarn v1 global cache at ~/Library/Caches/Yarn/ in clean_dev_npm()
• The existing ~/.yarn/cache/* only covers Yarn v2+ (Berry), missing the Yarn v1 (Classic) cache path on macOS
• This cache can grow to several GB over time (5.9 GB in my case) and is never cleaned by mo clean

Safety Review

• Does this change affect cleanup behavior? Yes — adds one new safe_clean target to the existing clean_dev_npm() function.
• Does this change affect path validation, protected directories, symlink handling, sudo boundaries, or release/install integrity? No. It uses the same safe_clean mechanism as all other cache paths, which respects whitelist and dry-run modes.
• The new path ~/Library/Caches/Yarn/* is a well-known macOS cache directory used exclusively by Yarn v1. It contains downloaded package tarballs that are re-fetched on demand, so deletion is safe and non-destructive.

Tests

• Verified ~/Library/Caches/Yarn/ exists on a macOS machine that has used Yarn v1
• Confirmed mo clean --dry-run does not currently list this path
• Confirmed the change correctly adds it to the dry-run output after patching
• Verified whitelist and --dry-run are respected (uses existing safe_clean helper)

Safety-related changes

• None. Uses the same safe_clean function as all other cache cleanup targets.