Closes #13245

Summary

Adds gh skill list to inventory installed agent skills across known agent host directories.

• Scans project and user skill install paths for all registered agent hosts
• Deduplicates shared install directories like .agents/skills
• Supports filtering with --agent, --scope, and custom scanning with --dir
• Renders a concise table with NAME, AGENT, SCOPE, and SOURCE
• Supports parseable output via --json, --jq, and --template with fields for skillName, hosts, scope, sourceURL, version, pinned, and path
• Records privacy-safe skill_list telemetry with filters, output format, and result count

Testing

• go test ./pkg/cmd/skills/list/...
• go test ./pkg/cmd/skills/...