Publish the Stack diagram agent skill alongside the CLI

[konojunya]

Summary

• Make the CLI repository the canonical home of the stack-diagrams Agent Skill.
• Document installation with npx skills add stack-sh/cli and reviewed local checkouts.
• Execute the skill command examples in process-level CLI tests and validate the rendered SVG.

The website usage guides will point here after this provider change is merged. The skill remains compatible with released CLI 0.4.0 and checks installed command capabilities before use.

Validation

• Skill frontmatter validator passed.
• CLI integration tests, cargo fmt, and git diff whitespace checks passed.