Add directory size caching

[SH20RAJ]

🎯 Goal

Cache directory sizes to avoid re-scanning and speed up repeated operations.

🚀 Features

cleanmac --use-cache        # Use cached sizes
cleanmac --refresh-cache    # Force refresh

📊 Example

✅ Using cached size information (scanned 2h ago)
📊 To refresh cache, use --refresh-cache

Cached data:
├─ Last scan: 2025-10-06 12:30:45
├─ Directories scanned: 245
├─ Total size calculated: 45.2GB
└─ Cache expires: 2025-10-06 18:30:45 (6h from scan)

🔧 Implementation Details

• Store cache in ~/.cleanmac/cache.json
• Include timestamps for invalidation
• Cache directory sizes and file counts
• Configurable expiry (default 6 hours)
• Detect if directories changed

✅ Acceptance Criteria

• Cache significantly speeds up scans
• Cache invalidates correctly
• Stale cache is detected
• Manual refresh works

🔗 Related

Part of Phase 5