Function validate_plugin_relative 

pub fn validate_plugin_relative(
    value: &str,
    root: &Path,
) -> Result<PathBuf, PluginError>

Validate that a plugin-relative path stays inside the plugin root.

Uses canonicalize (which resolves symlinks) so a symlink inside the plugin pointing outside (e.g. bin/link -> /etc) cannot smuggle a path out of the sandbox. The candidate must start with ./ per the Agent Plugins spec.

The target need not exist yet (a plugin may spawn a binary built at runtime), so when it is absent we canonicalize the deepest existing ancestor and verify the remaining suffix stays lexically inside the canonicalized root.