New plugin-cache-miss / refused-load error paths for claude.ai plugins

Claude Code now handles claude.ai-sourced plugins whose install record can't resolve. An 'unknown' registry read produces a plugin-cache-miss error; a 'present' record missing its origin pointer is refused with instructions to reinstall; and a non-installable plugin — files missing, no installable version listed, or an unreadable catalog — is refused with a message suggesting signing into claude.ai with the right organization.