Support generic git subplugin selectors

[nisiyong]

Description

Adds support for installing a specific monorepo sub-plugin from generic git URLs using repo.git#subplugin, matching the existing GitHub shorthand behavior.

Related issue:

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Documentation (if adding/modifying an adapter)

• Added doc page under docs/adapters/ (if new adapter)
• Updated docs/adapters/index.md table (if new adapter)
• Updated sidebar in docs/.vitepress/config.mts (if new adapter)
• Updated README.md / README.zh-CN.md when command discoverability changed
• Used positional args for the command's primary subject unless a named flag is clearly better
• Normalized expected adapter failures to CliError subclasses instead of raw Error

Screenshots / Output

npx vitest run --project unit src/plugin.test.ts
npm run typecheck

[nisiyong]

This feature is very important for enterprise use because we have a large number of plugins stored in a single, centralized repository.

Currently, our goal is for each "Skill" to correspond to its own specific CLI plugin. However, the current repository does not support installing individual sub-plugins. Every time we try to install a specific Skill, we are forced to perform a full installation of all CLI plugins.

This leads to significant environment pollution. We hope to collaborate with the community to build support for this specific feature.

[nisiyong]

@jackwener, I have integrated your two latest commits on my end, and the self-testing looks good.