bug: `claude plugin update` returns 'Plugin not found' for installed plugins

Bug Description

claude plugin update <name> returns an error even when the plugin is listed as installed by claude plugin list.

Steps to Reproduce

# Install a plugin from a custom marketplace
claude plugin install news-digest   # succeeds

# Verify it's installed
claude plugin list  # shows news-digest@news-digest as enabled

# Attempt to update
claude plugin update news-digest
# Output:
# Checking for updates for plugin "news-digest" at user scope…
# ✘ Failed to update plugin "news-digest": Plugin "news-digest" not found

Expected Behavior

claude plugin update should find and update the plugin, or at least explain that it's already at the latest version.

Actual Behavior

Returns exit code 1 with "Plugin not found", even though claude plugin list clearly shows the plugin as installed and enabled.

Workaround

Manually uninstall, delete the plugin cache directory, then reinstall:

claude plugin uninstall news-digest
rm -rf ~/.claude/plugins/cache/news-digest
claude plugin install news-digest

Environment

• Claude Code version: 2.1.90
• OS: macOS 15.3 (Darwin 25.3.0)
• Plugin type: custom marketplace plugin (single-repo marketplace, extraKnownMarketplaces in settings.json)