Add Slack cache as a junk category

[Vortrix5]

Summary

Slack (Electron) keeps Chromium caches under %APPDATA%\Slack (Cache, Code Cache, GPUCache, Service Worker\CacheStorage). Safe to clear, very similar to the existing discord-cache category. Leave session data (Local Storage, IndexedDB) so users stay logged in.

Where

src/sifty/core/junk.py, base dir %APPDATA%. The discord-cache category is a good template.

Acceptance criteria

• Category appears in sifty junk scan
• Only the cache dirs, never login/session data
• Sandbox test in tests/test_junk.py
• pytest green

Tip

Use the /new-junk-category skill.