Plugin cache not cleared on uninstall/reinstall, wrong version loaded

Bug Description

When uninstalling and reinstalling a marketplace plugin, the plugin cache directory is not cleaned up, and the plugin system loads an older cached version instead of the version specified in marketplace.json.

Steps to Reproduce

1. Have a marketplace plugin installed (e.g., review-pr at version 1.0.0)
2. Update the plugin's skill files in the source repository
3. Update marketplace.json to bump the version (e.g., to 1.1.0)
4. Run /plugin update or uninstall/reinstall the plugin
5. Invoke the skill

Expected Behavior

• The plugin cache should be cleared when uninstalling
• The latest version from marketplace.json should be downloaded and used
• The skill invocation should load from the correct version directory

Actual Behavior

• Old cached versions remain in ~/.claude/plugins/cache/<marketplace>/<plugin>/
• The skill loads from an older version (e.g., 1.0.0) even though marketplace.json specifies 1.1.0
• The system message shows: Base directory for this skill: /Users/.../.claude/plugins/cache/.../1.0.0/...

Evidence

$ ls -la ~/.claude/plugins/cache/<marketplace>/<plugin>/
drwxr-xr-x@ 4 jeremy.loy  staff  128 Feb 26 12:30 .
drwxr-xr-x@ 3 jeremy.loy  staff   96 Feb 25 15:54 ..
drwxr-xr-x@ 7 jeremy.loy  staff  224 Feb 26 12:30 1.0.0  # Old version still present
drwxr-xr-x@ 6 jeremy.loy  staff  192 Feb 26 12:30 1.1.0  # New version present but not used

Marketplace shows 1.1.0:

{
  "name": "<plugin>",
  "version": "1.1.0",
  ...
}

But skill invocation loaded from 1.0.0 cached directory.

Workaround

Manually bump the version number higher (e.g., to 1.2.0) to force a fresh download.

Environment

• Platform: macOS (Darwin 25.3.0)
• Claude Code: Latest version
• Plugin type: Local marketplace plugin

Impact

• Plugin developers cannot iterate on their plugins without manually clearing cache
• Users don't get the latest version after updating
• Stale plugin versions accumulate in cache directory causing disk bloat
• Confusing developer experience when changes don't appear after update

Suggested Fix

1. Clear all cached versions for a plugin when uninstalling
2. When loading a skill, use the version specified in marketplace.json (not the highest/lowest found in cache)
3. Add a --clear-cache flag to /plugin update for forced refresh
4. Consider adding a /plugin cache clean command to manually clear stale caches