Plugin validation now reports commands that reuse built-in names

Plugin validation is the check Claude Code runs to find problems in a plugin. It now reports an error for every command a plugin module registers under a name that Claude Code already ships.

Before, such commands were quietly filtered out and no error was shown. Claude Code still refuses to register these commands with $.command.register, and the hook that tries to register one is skipped. A hook is a piece of a plugin that runs at a set point.