add progressive disclosure support for Agent Skills

[jgpruitt]

• Add path parameter to view_skill tool for loading specific files
• Add availableFiles and metadata fields to skill output
• Implement directory scanning for scripts/, references/, assets/
• Add path validation with security checks (traversal, absolute paths)
• Update Zod schemas with metadata and availableFiles support

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[CLAassistant]

All committers have signed the CLA.

[murrayju]

This is superseded by #66