Add VS Code cache as a junk category

[Vortrix5]

Summary

VS Code keeps Chromium-style caches under %APPDATA%\Code (Cache, CachedData, Code Cache, GPUCache) that grow steadily and are safe to clear. Add them as a junk category.

Where

Follow the existing app-cache categories in src/sifty/core/junk.py. Base dir is %APPDATA%.

Acceptance criteria

• New category appears in sifty junk scan
• Only the cache dirs above, never the User folder (that holds settings/extensions state)
• Sandbox test in tests/test_junk.py
• pytest green

Tip

Use the /new-junk-category skill. Note Code stores settings under %APPDATA%\Code\User, so be careful to scan only the cache subfolders.