Improve skill standards and validation

[ivancasco]

Summary

• Normalize skill frontmatter with license and compatibility metadata.
• Fix RustFS client guidance for credential rotation and AWS CLI path-style addressing.
• Split the large use-slicer skill into focused reference files for progressive disclosure.
• Add a GitHub Actions validation workflow for Claude plugin validation and Agent Skills metadata checks.
• Tighten descriptions and validation rules so skill metadata is easier to trigger and references stay one level deep.
• Pin workflow actions to immutable SHAs with pinact.

Validation

• claude plugin validate . (passes with a marketplace-description warning because the marketplace metadata fix is intentionally split into PR Fix Claude plugin skill registration #3)
• Agent Skills metadata shell check
• yq . .github/workflows/validate.yml
• git diff --check

Scope

This PR intentionally excludes the Claude marketplace registration and README install-docs fix. That smaller fix is split into PR #3.