Create reusable GitHub Action workflows for Slide Spec

[lreading]

Summary

Post-v1 backlog item: create reusable GitHub Action support for Slide Spec workflows.

Potential scope:

• Package the CLI for action usage.
• Optionally run fetch when a token is available.
• Validate content.
• Build static output.
• Optionally scaffold or support deployment to GitHub Pages.

Alternatives considered

Keep action examples in docs until the CLI workflow is stable enough to package as an action.

Affected packages

• app (presentation renderer)
• cli (@slide-spec/cli)
• docs (documentation site)
• shared (types and validation)
• Unsure