feat: add cache management command

[JosXa]

Summary

• Add opencode cache command for managing the plugin/package cache directory
• Includes cache clean to remove cached plugins and cache info to inspect cache contents

This helps users diagnose and resolve slow startup issues caused by stale cache entries, where outdated plugin versions in the cache trigger force reinstalls on every launch.

Usage

# View cache info (path, size, installed packages)
opencode cache info

# Remove cache (with confirmation)
opencode cache clean

# Remove cache without confirmation
opencode cache clean --force

# Preview what would be removed
opencode cache clean --dry-run