Function deriveSubProjectRelativePath

src/node/services/systemMessage.ts:430–436

* Compute the path of `subProjectPath` relative to `projectPath` for use under * the workspace's own checkout. Returns `null` if the recorded sub-project * path is not actually a descendant of the parent project (stale persisted * state) — callers should treat that as "no sub-project segment" and

(projectPath: string, subProjectPath: string)

 * back to parent-only instructions rather than failing.
 */
function deriveSubProjectRelativePath(projectPath: string, subProjectPath: string): string | null {
  const relative = path.relative(projectPath, subProjectPath);
  if (!relative || relative.startsWith("..") || path.isAbsolute(relative)) {
    return null;
  }
  return relative;
}

/**
 * Read instruction sets from global and context sources as a structured tree.