Add a setting to disable specific built-in slash commands

Problem

User-defined slash commands in ~/.claude/commands/<name>.md shadow built-in commands of the same name at invocation time, but both still appear in the slash-command picker and in the assistant's available-skills list. This creates a confusing duplicate (for example, two /review entries) with no way to hide the built-in.

Request

A settings.json field, for example disabledBuiltinCommands: ["review", "init", "security-review"], that hides the listed built-ins from the picker and the skills list. Shadowing behavior would stay the same; this would just clean up the surface.

Why

• Power users routinely override /review, /init, and similar with repo-tailored versions.
• The duplicate listing is a steady source of "which one runs?" confusion.
• Today there is no workaround short of renaming the user command, which breaks muscle memory.

Environment

• Claude Code 2.1.142 (Homebrew cask)
• macOS 15 / Darwin 25.5.0