/plugin isn't available in this environment — error should guide users to desktop app or manual install

Summary

When a user runs /plugin install <package> in an environment where /plugin isn't supported (e.g. the claude.ai web interface or a remote CCR session), they see:

/plugin isn't available in this environment.

This gives no actionable next steps.

Proposed improvement

The error message should tell users what to do next, for example:

/plugin is only available in the Claude Code desktop app or CLI. To install a skill manually, place the SKILL.md file at ~/.claude/skills/<skill-name>/SKILL.md and restart Claude Code.

Why this matters

Skills published to private registries (e.g. @mercor/experimental-*) are being shared with team members who may be accessing Claude through the web interface rather than the desktop app. The current error leaves them stuck with no path forward — they have to reach out to the skill author to find out what to do.

Affected surfaces

• claude.ai web interface
• Claude Code Remote (CCR) sessions

Working surfaces

• Claude Code desktop app
• Claude Code CLI