Audit cross-platform CLI compatibility before v1

[lreading]

Summary

Audit Slide Spec CLI workflows for cross-platform behavior before v1.

The goal is that normal usage works on Linux, macOS, and Windows without Unix-only assumptions in paths, shell commands, file permissions, environment handling, or process management.

Acceptance criteria:

• Review code paths that touch filesystem paths, executable commands, environment variables, and dev-server behavior.
• Run at least a focused Windows smoke test if possible.
• Fix any platform-specific assumptions found during the audit.

Alternatives considered

Relying on Linux CI alone is not enough for a v1 CLI intended to work everywhere Node runs.

Affected packages

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