bug: /plugin update doesn't activate new plugin versions (new version cached as orphaned)

Summary

/plugin update pulls the latest marketplace but does not update installed_plugins.json to the new version. The new version is downloaded into cache but immediately marked as orphaned, leaving the old version active.

Steps to Reproduce

1. Install a plugin: /plugin install code-convention@wedatalab-plugins (installs v0.3.0)
2. Release a new version to the marketplace (v0.3.1)
3. Run /plugin update
4. Check ~/.claude/plugins/installed_plugins.json → still points to v0.3.0
5. Check ~/.claude/plugins/cache/wedatalab-plugins/code-convention/ → v0.3.1 directory exists but has .orphaned_at marker

Expected Behavior

After /plugin update, installed_plugins.json should point to the latest version (v0.3.1), and the new cache should be active.

Actual Behavior

• New version cache is created but marked orphaned (.orphaned_at file present)
• installed_plugins.json retains the old version and old installPath
• /plugin install after update says "already installed" — no upgrade path

Workaround

Manually edit installed_plugins.json and remove .orphaned_at from the new version cache:

# Remove orphaned marker
rm ~/.claude/plugins/cache/<marketplace>/<plugin>/<new-version>/.orphaned_at

# Update registry
python3 - << 'PYEOF'
import json
path = "~/.claude/plugins/installed_plugins.json"
# ... update version and installPath manually
PYEOF

Environment

• Plugin: code-convention@wedatalab-plugins
• Old version: 0.3.0
• New version: 0.3.1
• Marketplace source: github:WeDataLab-AI-Lab/claude-plugins