Skills and commands are now marked as builtin to resolve name collisions

What

• The reload_skills response the CLI sends now includes a builtin: true field on each skill entry whose resolved name matches one of Claude Code's own bundled skills.
• The schema describing an available skill or command gained an optional builtin boolean, true only for Claude Code's own built-in commands, used to decide which command a bare /name invocation runs when a user, project, plugin, and MCP command all share the same name.

Why

This removes ambiguity when a skill or command name collides with one of Claude Code's own: it's now possible to tell that a /name invocation is running the built-in version rather than a same-named user, project, plugin, or MCP one.