Implements skills support as described in #1540

What this PR does

This PR implements skills support for the vlt package manager, adding:

1. vlt skills command - A new CLI command that can:

   • List discovered skills (vlt skills list or vlt skills)
   • Link skills to project directory (vlt skills link)
   • Unlink skills (vlt skills unlink)

2. --allow-skills config option - Similar to --allow-scripts:

   • Uses DSS query syntax for allow/deny lists
   • Patterns like * (allow all), :not(*) (deny all), or specific selectors
   • When set, install automatically links skills after reify

3. Integration with install - After reify completes, if --allow-skills is set, discovers and symlinks skills from installed packages

How it works

• Discovery: Scans packages for skill files: skills.md, SKILL.md, *.skill.md
• Linking: Creates symlinks in ./skills/<package-name>/ pointing to skill files
• Filtering: Uses existing DSS query infrastructure for package selection
• Safety: Non-blocking - skills linking failures don't break installs

Implementation details

• Added skills.ts module in @vltpkg/graph with discovery and linking logic
• Created skills command in CLI SDK following build command patterns
• Integrated with existing bin-linking and reify infrastructure
• Added comprehensive tests for both CLI and graph functionality

Examples

# Install with skills linking enabled for all packages
vlt install --allow-skills=*

# List discovered skills
vlt skills list

# Link skills from specific packages
vlt skills link --target="#express, #lodash"

# Remove skill links
vlt skills unlink

Closes #1540