Function resolve_cwd 

pub fn resolve_cwd(
    cwd: Option<&str>,
    plugin_root: &Path,
    plugin_data: &Path,
) -> Result<PathBuf, PluginError>

Resolve an explicit or default cwd after placeholder expansion and enforce post-resolution containment.

• None → canonical plugin root (spec default).
• ./... or ${PLUGIN_ROOT}[/...] → must stay within the canonical plugin root.
• ${PLUGIN_DATA}[/...] → must stay within the canonical plugin data dir.