[FEATURE]: Desktop client should provide cache management and cleanup settings #29037

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

opencode generates various cache files under ~/.local/share/opencode/ during normal operation. The tool-output/ directory, which stores raw output from tools like WebFetch (HTML, JS, cookies, etc.), can grow to 20GB+ over time with no automatic cleanup mechanism.

Currently, compaction settings (compaction.prune) only manage in-memory context/token compression and do not affect on-disk cache files. Users must manually rm -rf these directories.

Suggested solution:

Add a cache management page in the Desktop client's Settings/Preferences with:

1. Cache overview — list cache categories and their current disk usage:

   • tool-output/ — tool output cache (WebFetch, etc.)
   • log/ — log files
   • repos/ — cloned repositories
   • storage/ — session storage data
   • Session/prompt history

2. Cleanup actions:

   • One-click "Clear" button per category or "Clear All"
   • Auto-cleanup policies (e.g., on exit / by size threshold / by age)

3. Storage limit — configurable maximum cache size; auto-clean oldest data when exceeded

Benefits:

• Prevents unbounded disk usage without user awareness
• Improves desktop client user experience
• Aligns with common IDE conventions (e.g., VSCode's built-in cache management)