Add package-manager download caches (npm, pip, yarn) as a junk category

[Vortrix5]

Summary

Developer package managers keep large download caches that are safe to clear and re-fetched on demand: npm %LOCALAPPDATA%\npm-cache, pip %LOCALAPPDATA%\pip\Cache, yarn %LOCALAPPDATA%\Yarn\Cache. Fits Sifty's developer-cleanup angle.

Where

src/sifty/core/junk.py, base dir %LOCALAPPDATA% (use the existing _local_appdata() helper).

Acceptance criteria

• Category appears in sifty junk scan
• Only cache dirs (these hold no config or auth)
• Sandbox test in tests/test_junk.py
• pytest green

Tip

Use the /new-junk-category skill. One category covering all three is fine.