[Bug] Built-in plugin startup tip references unavailable plugin #99071

Bug Description
A startup tip suggests running:
/plugin enable cc-plugin-you-should-know@builtin
("Want a side agent watching your back while Claude works...")

Following it fails:

• /plugin enable cc-plugin-you-should-know@builtin → "Plugin is not installed in this project"
• /plugin install cc-plugin-you-should-know@builtin → "Marketplace "builtin" not found"
• /plugin install cc-plugin-you-should-know → "not found in any marketplace"
• /reload-plugins didn't change anything

Expected: the plugin from the tip installs/enables, or the tip isn't shown if it's unavailable in my version.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.287
• Feedback ID: 2a4944e5-654c-43c7-a01d-0891d3e001fc