Add optional CLI accessibility checks after build

[lreading]

Summary

Post-v1 backlog item: explore a CLI accessibility check that can run after build.

Desired shape:

• Default to warnings rather than hard failures.
• Add --no-a11y to disable checks.
• Add --strict-a11y to fail on warnings.
• Avoid bloating the CLI or making builds unexpectedly slow.

Alternatives considered

Keep accessibility checks in app/docs CI only until there is a lightweight CLI path.

Affected packages

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