[Bug] `/plugin install` slash command ignores @marketplace qualifier for plugin resolution

Bug Description
/plugin install <name>@<marketplace> ignores the @marketplace qualifier
when two marketplaces expose plugins with identical names.

Repro:

1. /plugin marketplace add remember-md/marketplace
2. /plugin install remember@remember-md

→ installs remember from claude-plugins-official instead

Expected: @remember-md is authoritative (as docs state and as
claude plugin install remember@remember-md from the CLI does).
Actual: slash command picks the first name match across marketplaces
and silently drops the @marketplace suffix.

CLI works correctly. Only the slash command is affected.

Environment Info

• Platform: darwin
• Terminal: vscode
• Version: 2.1.92
• Feedback ID: c87c8665-d945-47e0-9579-46abcfca10ab