feat: add --all flag to skills add for bulk install

[christso]

Summary

When setting up a workspace with all skills from a popular repo (e.g., rstackjs/agent-skills with 21 skills), users must install each skill individually. A bulk install option would simplify this.

Current behavior

# Must install each skill individually
allagents skills add rsbuild-best-practices --from rstackjs/agent-skills
allagents skills add rspack-best-practices --from rstackjs/agent-skills
allagents skills add migrate-to-rsbuild --from rstackjs/agent-skills
# ... 18 more

Desired behavior

# Install all skills from a source at once
allagents skills add --all --from rstackjs/agent-skills

npx skills equivalent

npx skills add rstackjs/agent-skills --all

This installs all 21 skills to all configured agents without prompts.

Implementation

Add --all flag to skills add command:

1. When --all is set with --from, discover all skills in the source
2. Enable all discovered skills (skip allowlist filtering)
3. Add all skill names to the plugin entry in workspace.yaml
4. Run sync

Related

• feat: add --list flag to skills add for remote skill discovery #368 (--list flag for discovery)
• npx skills: npx skills add <owner/repo> --all