Support skill dependencies via frontmatter `depends` field

[serejke]

Problem

Skills can reference sibling skills via relative paths (e.g., Read ../gws-shared/SKILL.md), but npx skills add has no awareness of these references. When a user selectively installs a single skill, prerequisite skills are silently missing, leaving broken references.

Reproduction

npx skills add https://github.com/googleworkspace/cli/tree/main/skills/gws-drive -a claude-code -y

This installs only gws-drive/SKILL.md, which contains:

PREREQUISITE: Read ../gws-shared/SKILL.md for auth, global flags, and security rules.

But gws-shared is never installed. The AI agent following the skill instructions hits a dead reference.

Proposal

Add an optional depends field to the SKILL.md frontmatter spec:

---
name: gws-drive
description: "Google Drive: Manage files, folders, and shared drives."
depends:
  - gws-shared
---

When npx skills add installs a skill with depends, it should automatically resolve and install the listed skills from the same source repository. This is analogous to how package managers resolve peerDependencies.

Current state

• The frontmatter spec supports only name, description, and metadata
• No dependency resolution exists — skills are treated as fully independent
• Skill authors work around this with prose instructions ("Read ../gws-shared/SKILL.md"), which are invisible to the tool