feat: add --list flag to skills add for remote skill discovery

[christso]

Summary

When installing skills from a remote source, users need to know what skills are available before committing to an install. Currently, allagents requires installing the plugin first to discover its skills.

Current behavior

# Must install first to see what is available
allagents skills add rsbuild-best-practices --from rstackjs/agent-skills

Desired behavior

# List available skills without installing
allagents skills add --list --from rstackjs/agent-skills

# Output:
# Available skills in rstackjs/agent-skills:
#   migrate-to-rsbuild     Migrate webpack, Vite, CRA projects to Rsbuild
#   rsbuild-best-practices Rsbuild best practices for config, CLI, etc.
#   rspack-best-practices  Rspack best practices for config, CLI, etc.
#   ...

npx skills equivalent

npx skills add rstackjs/agent-skills --list

This shows all 21 skills in the repo with descriptions, without installing anything.

Implementation

Add --list / -l flag to skills add command in src/cli/commands/plugin-skills.ts:

1. When --list is set, fetch the source but do not install
2. Discover skills using existing discoverSkillNames() function
3. Read SKILL.md frontmatter for descriptions
4. Display skills with names and descriptions
5. Return without modifying workspace.yaml or running sync

Related

• feat: add lock file for reproducible remote plugin syncs #335 (lock file for reproducible syncs)
• npx skills: npx skills add <owner/repo> --list